There are gigantic flower arrangements in red, white and blue pinned to the walls. A circular sign on the podium announces, "Inauguration of the President and Vice-President of the United States of America".

This is not, of course, the real thing. We are on location for The West Wing, the hugely successful television series, written by Aaron Sorkin, about life behind the scenes in the White House. (Normally the show is shot in Los Angeles on a set with a replica Roosevelt Room and an Oval Office emblazoned with presidential seals.)

The West Wing is a well-acted and finely written show, but it is utterly exhausting to watch.  Every week there is a fresh threat to world security and another crop of vital issues of state hang in the balance while the very busy, super-smart, highly principled workaholics that make up the president's staff stride briskly from office to office talking at speed in slick, informative gobbits. No one in The West Wing ever says, 'I dunno,' or scratches their crotch, or fails to come up with a witty rejoinder. No one ever sleeps.

But American audiences love The West Wing and the series, which is now entering its fourth year, has won 22 Emmy awards and has regular viewing figures of 16 to 18 million. Here in Britain, where we can't muster the same political idealism (an equivalent high drama set in Downing Street and featuring an idolised Alastair Campbell is unimaginable), the show's increasing popularity - two million viewers watched the last series on Channel 4 - is somewhat more surprising.  The West Wing, after all, is a pure American product. And so, too, is its star - the 62-year-old actor Martin Sheen, who plays the thoroughly good and upstanding Democrat President Josiah Bartlet.

Sheen is now standing in the middle of the hall having an extended farewell chat with a departing radio interview team.  A short, compactly built man, he looks the picture of health and good cheer: shiny eyes, light suntan, nicely capped front teeth. He has a very straight profile: a straight chin and upper lip, and a high forehead accentuated by the fact that his hair rises straight up continuing the same line.

In Terrence Malick's Badlands (1973) or Francis Ford Coppola's Apocalypse Now (1979), the two famous films where Sheen, then in his thirties, played strange, sociopathic outsiders, his eyes were troubled and his hair an unruly mop.  Nowadays he has broadened out. His eyes are twinkly, as if he is always just about to smile. And his coiffure couldn't be neater - the glossy hair swept up and back in a perfect dome. This transformation is what marks Martin Sheen out from his peers. Look at how other big Hollywood stars have aged.  Clint Eastwood, for example, has just become a more pared down, lined version of himself.  Al Pacino has just become more and more dishevelled and anguished.  But nowadays Martin Sheen looks like a quite different sort of man.

Along with the twinkliness comes very good, slightly laborious manners. He is most affable, asking everyone's name and where they are from and how their journey was, and when they got in to Washington.  As he speaks he fixes his gaze fully on each person in turn. However he is still a little distracted and, it is clear from some of his follow-up questions, he isn't quite taking in the replies.

We go upstairs to a quiet hallway, followed by Sheen's retinue - a publicity woman, a harassed-looking make-up assistant with a zip-up plastic bag of combs and lotions and a silent, dark-haired youth called Taylor who is one of Sheen's grandchildren and works as his personal assistant.  Sheen checks his face out in the mirror, held up by the harassed-looking make-up assistant. The dome of hair is still glossy and perfect, but Sheen exclaims, 'What a ragamuffin!' And, with that disconcerting facility that actors have, he chats blithely as the make-up assistant prods at his head with a four-pronged comb.

Behind him a red, white and blue flag flutters out on the balcony. It must, I suggest, feel a little odd to be acting the president, to have all those same flowers and flags and retainers and press attention, when the real thing is so close by. Sheen shakes his head and says, 'No I have never had that illusion. Never, thank God. I hope I never will.'  I hadn't suggested he would be so deluded, but Sheen is still concerned about this possibility and a minute later repeats himself. 'It's a wonderful show. I am enjoying it. I am having the time of my life. I couldn't be happier, but I am not at all confused about who I am vis-à-vis Bartlet.'

The West Wing has provided Sheen's career with a second wind. After decades of playing relatively obscure parts the actor has re-emerged as a superstar. He has been fêted by Democrat politicians - Bill Clinton and Al Gore have both visited the set - and been the recipient of a vast salary estimated at around $6,600,000 a year, which is 16 times more than the salary of the real President of the United States.  Meanwhile the highlights of the series - such as the assassination attempt on President Bartlet, or his confession that he suffers from multiple sclerosis - have become public talking-points.

At first, Sheen says, he tried playing the part with a certain presidential grandeur.  'I sort of started with pomposity and self-importance and he [Aaron Sorkin] said, "No, no, no, this president is whoever you are." So now I never have to play the president. They [the rest of the cast] treat me like one and that is what sells it.'

Originally Bartlet wasn't going to be such a big part in The West Wing and the series was designed with the president only appearing in one show in four. Sheen was brought in as an afterthought - he only got the part after James Earl Jones and Sidney Poitier had turned it down. But in the first pilot the actor stole the show.  Sheen seemed made for the part, and his character could combine moral probity and high-mindedness with great personal charm. So Sheen's part became bigger. And President Bartlet became somewhat Sheen-like, adopting many of his beliefs and foibles.  Bartlet is a practising Catholic, like Sheen. Bartlet is obsessed with the large, big-brush moral questions, as is Sheen. Bartlett is irascible and Sheen says he, too, is easily exasperated. Bartlett loses things, as does Sheen. Bartlet is forgetful, Sheen says he is terrible at learning lines.

He guffaws, 'I am infamous. Infamous! Aaron Sorkin doesn't write normal dialogue. It's very complex stuff. There are lots of references and words that I can't pronounce, let alone know what they mean.'  At the very end of the interview, when we have talked about everything from his struggle with alcoholism to what a wonderful grandson he has, the actor embraces me affectionately. Then he asks, 'What's your name again?'

Sheen sees his President Bartlet as an amalgam - the very best elements of Presidents Kennedy, Carter and Clinton brought together and poured into one man. Yet his Bartlet is far more reminiscent of the folksy fireside charm of George W Bush.  This comparison would probably mortify Sheen, who is far to the left of any Democrat president, let alone Bush whom he describes as a 'man out of his depth'. Sheen is a pacifist and vehemently opposed to war against Iraq. 'This war is so heinous!' he exclaims. 'All wars are heinous, but this is so obviously a political manoeuvre.'

The actor's political radicalism feels like an offshoot of his religious faith. He doesn't discuss power shifts, or the rise of Islamic fundamentalism, or the price of oil. Instead, he talks in slow, measured tones about the human cost: the men, women and children. And as he talks he rocks back and forth on his chair.

'This war will result in horrible death and destruction of the Iraqi people who are the most innocent of all. Many women and children will die, many more will be traumatised. And then the effect on our own soldiers In the American military, there are a lot of women, a lot of married women and mothers now [his voice has gone down to a whisper]. That is going to make a profound difference.'

Does he wish that he was going to Iraq with the American peace activists, the so-called 'human shields'? 'I have great, great admiration and respect [for them] - I don't have that kind of courage unfortunately. I am a big coward when it comes to confronting violence.'

Sheen is, however, an active campaigner, supporting poor and disenfranchised groups and demonstrating at anti-nuclear rallies and against the death penalty. He has been arrested more than 70 times and is still on probation after demonstrating against the 'Star Wars' missile defence system outside an air-force base in California in 2000.  Last week he spoke at a peace march in San Francisco. 'I participated with Joan Baez and the Congresswoman Barbara Lee - an extraordinary woman.' He shakes his head and adds a little dreamily, 'See. It's the women that will save us. I am serious. It is the women. Dostoevsky said, "the world will be saved by beauty". He meant the women, I think.'

Sheen's speech frequently assumes these vague, slightly mythic tones. Take, for example, Sheen on Sheen's childhood. The actor was born Ramon Estevez, one of ten children of an Irish mother and a Spanish immigrant factory worker in the Midwest industrial town of Dayton, Ohio.  Almost the first thing Sheen says is, 'I was a seventh son' - a reference to the superstition that seventh sons are marked out as lucky and especially graced by God.

It was otherwise an inauspicious beginning.  'My mother had a very difficult birth with me, however. They had to use forceps and in the emergency they crushed my left shoulder and I was not breathing very much, and they kind of threw me aside to work on my mother, who was dying. They baptised me there because they thought I was finished.

'My mother died when I was very young and I started working when I was nine. I started caddying at the local exclusive country club. It had such a profound impact on me. I could never to this day belong to a private club - I can't. I love to play golf - I don't play that well and I don't play that often - but I will only play on public courses I can't belong to a private exclusive anything. That is what that experience taught me. That would limit me to people like me - overprivileged.'  Here Sheen stretches out his arms in a great, expansive gesture: 'I want to be exposed to all of humanity. I want all of humanity to be reflected in me. I want to know what it is like to be human.'

Sheen's father struggled to keep the family together. He instituted rotas for chores and cleaning the little three-bedroom house, and sent the children to a strict Catholic high school. Sheen laughs nostalgically, 'My poor Dad! It was a real trial for him. He was easily the best man I ever knew - honestly, to this day. The most honest and the most loyal.'

Nine of the ten children were boys, and they mostly went into the services. But this wasn't an option for young Ramon, because of his misshapen shoulder. (It doesn't in fact look too bad, but Sheen still has restricted movement in the left arm, and it is three inches shorter than the right arm.)  His father had saved up for him to go to college, but Sheen was an inattentive scholar. 'He wanted me to go to college. I knew it was a waste, but he didn't understand [whispered tone] I had to explore myself. I had to become myself. I had to have the freedom to become myself.'

So at 18 he went to New York to become an actor?

'No,' Sheen corrects me, 'I didn't go to New York to become an actor. I was an actor already.'  He shrugs as if stating the obvious, 'I never was not an actor. There was something inside of me. I don't know what it was but when I was six or seven and I started to go to the movies and eventually it occurred to me, "Oh, I am one of them. I can do what those people up on the screen do. I can do that," and I didn't know why or why not. I would enact the movies on the way home of course. All of us did, I loved some characters and I protected them and they lived in the book and volume of my heart of course. Jim Cagney.'  He pauses, savouring the memory. 'Hmm...Jimmy. And then, you know, Spencer Tracy. There were so many actors that I loved.'

Sheen is settling in. He has now swivelled round in his chair, removed a moccasin and has stretched one foot out against the wall. His voice has taken a faraway tone.  'Then as a teenager James Dean happened and that changed everything. Oh my God! One of us got through! And that guy wasn't acting. All of us wanted to be James Dean. Everybody loved him because he was like everybody...  He transcended cinema acting like no one before him - with the possible exception of Marlon Brando - because he no longer acted. Or at least the image, and the perception was that he wasn't acting. It was behaviour.'

In New York Sheen soon started working with the Living Theatre, an avant-garde performance group of committed pacifists. He also took on his new name, Martin, from the casting director Dale Martin, and Sheen after Fulton J. Sheen, an evangelising bishop on television whom he very much admired.  Sheen is still very insistent that he never officially changed his name - it's just a stage name. It's another of his slightly eccentric points of principle, like not joining private clubs. He brings out his driving licence just to prove his case.

In 1961, when he was 21, Sheen married Janet Templeton, an art student, and, over the next six years, the couple had four children: Emilio, Ramon, Charlie and Renée. Sheen was becoming a relatively respected young actor - he had 15 months as the lead in the Broadway hit The Subject Was Roses.  By 1970 he had gravitated to Los Angeles, buying the house in Malibu where he and Janet still live.

Sheen starred with Sissy Spacek in Badlands, Terrence Malick's 1973 film based on the true story of an adolescent who went on a killing spree.  At 32, Sheen was technically too old for the part of the deluded and desperate Kit Carruthers. However, Sheen looked much younger than his years, and his portrayal of repressed turbulence and then the increasing madness of his character is completely convincing.

Badlands is now acknowledged to be a masterpiece, but at the time the film went virtually unnoticed.  Instead Sheen got his big break three years later, in 1976, when Francis Ford Coppola (who had rejected him for the role of Michael Corleone in The Godfather [1972]) cast him as the lead in Apocalypse Now, his Vietnam epic based on Joseph Conrad's Heart of Darkness.

Sheen played Captain Willard, a tormented military assassin and the narrator of the film. The shoot, in equatorial swamps in the Philippines, was long, hellish and increasingly crazy. Sheen was smoking three packets of cigarettes a day and drinking heavily, eventually descending into a state of desperate inner turmoil.

On his 36th birthday Coppola asked him to improvise the opening sequence of the film, where Willard wakes up in a hotel room. During filming, Sheen, who was so drunk that he could barely stand up, lost himself completely in the part and gave an extraordinarily intimate and exposing performance. At one point the actor smashed the mirror in front of him with his fist, cut his hand open and painted his face with his own blood. At the end of the shoot he passed out.

A year later, when he was still filming in the Philippines, Sheen had a major heart attack. He nearly died, had his last rites and the whole white-light-in-tunnel experience.

He had given surely everything to the part.

Sheen grins. 'Not quite. I didn't give my life.' Then, with a chortle, he adds, 'Damn near!'

So does he feel now that when he acts he keeps more of himself back?

'On the contrary, I give more, but I don't give more in the same way. I am more committed to my craft than I ever was...I'm infinitely more mature than I was then. I didn't know how to use myself and so I abused myself.  Because there was so much actually happening I didn't have to go very far. It was happening to me. All my fears were on the surface. All my doubts and despair were very, very evident - all of my self-loathing and self-destructive behaviour was happening on camera.'

Sheen says he sometimes comes across Apocalypse Now by accident when he is channel-surfing.  'I will see the movie and be captivated by it. But I was who I was, not who I am now. A lot of it was who I was nearly 30 years ago.'

After Apocalypse Now Sheen went through four more hard years of alcoholism and self-abuse, and at one point he left Janet and the children. But the marriage pulled through and Sheen describes his wife as 'a wonderful woman and my teacher'.

Finally, in 1981, Sheen seems to have found himself. He cut back on the drinking, returned to the Catholic Church and took up politics. The reformation didn't happen overnight. Sheen continued to struggle, and it was only 11 years ago that he finally gave up drinking altogether and joined Alcoholics Anonymous.  Recently, after 14 years of abstinence, he started smoking again. He has now quit again but says it's hard.

The bad years took their toil on Sheen's family. He says his children 'didn't have an easy time. I was focused on a large measure on myself. I didn't know they had certain needs.'  Two of his sons, Emilio and Charlie, have had serious problems with drink, drugs and violence. In May 1998, Charlie Sheen, after a heroin overdose, suffered a stroke. Sheen says Charlie son is now 'clean and sober and happy as Larry.' And concerning his children, his only regret now is 'that we didn't have four more'.

After Apocalypse Now, one of the greatest films of the 1970s, Sheen kept a relatively low profile. He let it be known that he wouldn't do violent films and turned down Blade Runner (1982) - the part went to Harrison Ford. 'I wasn't focused on a career. I was trying to focus on staying alive.'

For 30 years Sheen then worked prodigiously, and has been in constant demand as a character actor in Hollywood. But, until The West Wing came along, he hasn't done work that has brought him acclaim, or been of anything like the same stature as Badlands or Apocalypse Now.

'I still haven't really got that sense of career,' Sheen says. 'I just don't have a handle on it.'  Perhaps the right part just never came along, but one cannot help wondering whether this actor, more than most, needed to be able to empathise with his characters.

Sheen needed someone as a big-hearted and sentimental as himself. He needed the part of a great romantic, an idealist. Someone to whom Big Things Happen. And who could be better than President Bartlet? Here is someone who also thinks in big broad brushstrokes and talks in great big quivering generalities. Like him Sheen is poor on detail, and maddeningly vague. But he feels. And when he feels, he glows.

After the interview Sheen still wants to talk, and we walk down on to the balcony. The camera lights are now blazing, and the hall below is crowded with about 200 film extras in ball-gowns and evening dress.

Sheen looks down. He says, 'It's nothing to do with me. It just all happens around me.'  Then we go downstairs, and Sheen makes his way to his dressing-room.

For the next couple of hours the extras practise. They are exhorted to imagine that this is the president's inauguration ball and they have all had a couple of drinks and are really enjoying themselves. Again and again they swirl round and round the room.

Eventually the actors start to arrive on the scene and I realise that Sheen must be somewhere close at hand because the harassed-looking make-up assistant with the zip-up plastic bag suddenly materialises by my side.

And then, here comes Sheen himself.  Fully made-up now and in immaculate evening-dress, he looks even more happy and shiny. There is a spring to his step and he now seems to have that weird static of a very famous person.

As he walks straight past us and out into the middle of the ballroom, he is met with a ripple of excited chatter and clapping. Sheen smiles enchantedly at the crowd and lifts one arm up into the air as if he is gathering up the applause.  He grins, swivels on his heel, bows down and departs.

A minute later it is time for the shot. The music starts up again, the extras dance around the room in crowded swirls. Then huge double doors open.

And Martin Sheen and his retinue walk briskly in.

President Sheen, a man of grandeur from the tip of his shiny black shoes to the crown of his glossy hair, never has to slacken his pace. The dancing couples part before him and he strides across the crowded ballroom like a knife through butter.

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