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The Biltmore in downtown Los Angeles is one of the kitschest hotels in California. And that's saying a lot. Composed as a Renaissance fantasy that leaves no cupid unarmed, there are chandeliers, mirrors, curving balustrades, painted ceilings, naked goddesses and sea monsters. Everywhere you look, there is more. Martin Sheen isn't kitsch, although he does admit he's a shameless ham. "I love being an actor," he says. "If I can make a living off fakery and haminess, I'll be as happy as Larry."

So here he is on the ninth floor. Quite short, quite loud, wearing Versace jeans and laughing a lot. Ramon, 36, his second child, works as his personal assistant. "Ramon! Please get me a cappuccino, you know I've always loved you ... He's a tyrant, he's only being nice because you're here ..." Ramon - dark, shy and thin - disappears downstairs in search of a Starbucks while his father reveals himself to be very funny, very principled and very in his element playing the President of the United States of America.

You still think of him as Willard in Apocalypse Now, sailing up the Nung River to meet Kurtz, the devil-god of dark Cambodia. It was a mesmerising performance and one that nearly killed him. Volatile and emotional, he was a wild man who drank and smoked and gambled and never knew how to stop. He collapsed with a heart attack in the middle of the Philippines jungle and was carried off the set on a stretcher. But that was more than 20 years ago.

Sheen's 60 now and the star of The West Wing, a TV show that dramatises the lives of the people working in the Oval Office. Though usually filmed on a sound stage at the Warner Brothers studio, the scenes today are at the Biltmore and there is all the usual hanging around reading the National Enquirer and eating "wholesome" double chocolate-chip cookies. The crew is family, the family is crew. They eat together, play basketball together. "Martin talks to everybody," says Liza Croisette, a stand-in. "But I've noticed that he graduates towards the quiet ones."

The West Wing humanises the lives of the various aides, secretaries and speech writers who are the machinery of Congress. In the US, the show has collected a prime-time audience of about 20 million. One poll during the recent US elections showed that Sheen's character, Josiah Bartlet, would have swept the country with 75 per cent of the vote.

The West Wing has initiated a series of entertaining ironies, thanks in part to the ever-decreasing difference between politics and showbusiness, and to the fact that it was fortunate enough to run in the middle of a presidential election. Bill Clinton loves The West Wing, going so far as to suggest a story line.

Dubbed "The Left Wing" by those who don't like its Democratic bent, the series is written by Aaron Sorkin, a Hollywood screenwriter whose political leanings were crystallised at the age of 11. Campaigning for George McGovern, a veteran liberal trounced by Richard Nixon in 1972, Sorkin was on the street waiting for the motorcade when an old lady grabbed a sign from his hand and hit him over the head with it. He has said that a part of him has been trying to get back at her ever since.

Sorkin's President Bartlet is a liberal Democrat from New Hampshire, and the series, now in its second year in the US, understands that to dramatise this political stance is to imbue it with a dramatic heroism. The characters are human and imperfect and therefore very watchable. Idealistic without being worthy, they are dedicated to noble causes. The result is that the US is watching a primetime series that assumes it has liberal integrity.

"The conservatives are keeping an eye on us," he says. "They are curious about why the program is so popular when the country is so conservative. It is like watching a forbidden fruit being consumed."

Sheen is having the time of his life. Not only is he enjoying playing Bartlet, who is homey and human and the linchpin of the show, but he also appreciates the wider effects that The West Wing is having. He believes, in particular, that the show will attract an interest in public service, a profession that needs more intellects than it attracts. "It is the moral references that are keeping people out," he says. "They don't want their private lives known. But we all have our humanity to deal with whether we are public servants or not. I would never have blamed Mr Clinton for his sexuality, but I would blame him for lying."

Sheen has been a voluble left-wing activist for many years, pursuing a rigorous schedule of disapproval that began in the early 1980s when he met Dan Berrigan, a Catholic priest whose dissent had incurred regular prosecution. In 1986, Sheen joined Berrigan on a demonstration against Reagan's Star Wars program and was arrested. It was, he said later, the happiest day of his life.

He has come out for strawberry workers and against nuclear testing, for gun control and against US involvement in Nicaragua. He is an environmentalist and a pacifist. In 1992, he was arrested outside a factory that supplied parts for nuclear reactors, and faced trespass charges after a demonstration against a hazardous-waste incinerator. In 1995, he was arrested outside the Pentagon and, in the same year, was forced to hide out in a church in Canada to avoid a mob of angry seal hunters on a drunken rampage.

He has been asked to consider an official career in politics, but has long vetoed the idea. Martin Sheen does not want to be a real president. "The republic is safe," he laughs. "I don't have a personal interest in politics per se. I have a great interest in the issues that are publicly debated, but I have a far greater interest in social justice and peace. I could never be free to explore that if I was bound to a constituency. If I was bound to a constituency I would have to foreclose my principles."

Principles, though, are complicated things, entangled as they are in webs of anomaly. The West Wing is broadcast by US network giant NBC. NBC is owned by General Electric, a vast multinational with a long history as one of America's leading defence contractors and of involvement in the nuclear power industry. Together they represent the type of corporate control that dominates the cultural mainstream and funds a dangerous level of self-interest in government: the very issues with which Sheen so vehemently disagrees.

These ironies are not lost on Sheen, nor have they stopped him from doing as he wishes. In August 1999, while working on The West Wing, he attended an anti-nuclear rally in New Mexico. "I have to be aware of what I am doing," he says. "It is hard to live in a Western culture now - make a living, have a bank account - and not be part of the oppression of the Third World." Yes, he could lose his job. "They could let me go tomorrow, but I can't worry about that."

Sheen is a complicated man. Luvvie anecdotes vie with political observations; wisecrack one-liners segue seamlessly into conversations about God, Hitler and the world's resources. Despite a yoga injury he enacts a full rendition of the time Fellini threw him off a set - a story that sees Sheen acting out all the characters, including Fellini, Ava Gardner and a bodyguard. You get "George Bush is a complete moron" followed by a tale about the time Marlon Brando came for dinner ("The children just thought he was a fat guy; they didn't know who he was"). Then "Did you see Billy Elliot? I cried three times", followed by "Can you tell me, is my hair a little askew?"

He loves to chat. It must be his heritage. His mother was Irish, his father Spanish. You can imagine him down the pub, telling stories, having a laugh. The seventh son of 10 children, he grew up "dirt poor" in Dayton, Ohio. His mother, Mary Ann, died when he was 11 and his father, Francisco Estevez, a drill press operator, brought up 10 children, nine of whom were boys. He loved his father, but in February 1959, aged 18, Sheen boarded a Greyhound bus and moved to New York to become an actor. He fell in with the Living Theatre, an avant-garde group of fringe performers, and changed his name from Ramon Estevez to Martin Sheen; the former made him vulnerable to innate racism, the latter was more matinee.

Having appeared in the TV series Hawaii Five-O and Mission: Impossible, he was cast by Terrence Malick in Badlands. In 1970, he moved to Malibu, where he still lives in the same house with his wife of 40 years, Janet, a former art student. They have four children: Emilio, 38, Ramon, 37, Carlos (Charlie), 35, and Renee, 33.

They are a close-knit but emotional family, living a public Hollywood life. Martin and Charlie, long embroiled in a tumultuous relationship, appeared in Wall Street in 1987, the former, as a union leader, conveying the film's voice of moral reason. Stockade, in 1989, saw Martin as a bullying sergeant attempting to pummel Charlie, as Private Bean, into submission. Both had some resonance with reality. By the age of 25, Charlie was out of control. He had charisma, acclaim, talent and money, but he was a drunken maniac. He owned BMWs, Porsches and a yacht, collected handguns, beat up a girlfriend, dated a porn star and, in 1998, after smoking crack for hours, he was rushed to hospital with a suspected heart attack. He escaped from rehab in his limo, at which point his father had him arrested.

Nowadays Charlie is credited with increasing the ratings of Spin City, the TV show in which he replaced Michael J. Fox. He is still flash - his trailer is a million-dollar climate-controlled customised bus, with a zebra bedspread - but nowadays he is described as a "reformed bad boy". It has been a long haul. "I had been praying for Charlie's sobriety for years," says his father. Charlie was surrounded by thugs and guns and his father thought he might die. He organised various "interventions" and at one time asked Clint Eastwood to help out, but Charlie just holed up and took coke. His father did not give up. "Sometimes the addict is not asking for help in language," he says, "but in their behaviour. I asked God to work through me and to show me the way, and that was the choice I had to make. Even though I was frightened, I had him arrested."

Sheen snr is a member of Alcoholics Anonymous, which he came to as a result of his Catholic faith. Yes, he felt guilty. Yes, he knew things had not been easy for his children. Excess had nearly killed him and the children had suffered from this. His wife and the younger ones had been with him on the set of Apocalypse Now when he collapsed. Fourteen-year-old Emilio had already fled to LA following a fist fight with his father, only to be called back to an emergency ward in Manila.

Charlie, aged 10, visited the hospital and was horrified but was the only one who did not cry. His father dates Charlie's paranoia and interest in automatic weapons to this troubled time. Charlie had smoked his first joint by the age of 11. Later, father and son would go to the racetrack and out drinking together. "I was dangerous, hurtful and very selfish," says Sheen. "We drink to kill the pain and to hide the reality that everybody sees. I think alcoholism could be passed on genetically, but I also think we pick up things without knowing that we are picking them up. We deposit our bad stuff as well as our joy to our children and they pick it up. I'm convinced of it."

Family life seems settled and is centred in Malibu. Sheen has three grandchildren: Taylor, 17, and Paloma, 14, are Emilio's children by a former girlfriend, Carey Salley. Cassandra, 16, is Charlie's daughter by Paula Profitt, who became pregnant at 17 when he was 19. There was talk of a termination but Sheen snr, who does not believe in abortion, intervened, setting up a trust fund and providing a house so as Profitt could keep the baby. Charlie, who did not have a relationship with Profitt, did not think it was his father's business. "The boys love the children," he said recently, "as I was sure they would when they had the chance to mature a bit."

Sheen's working day starts at 7am and often finishes very late. He rolls through it with unflagging energy. "It ends when it ends," he says. "Where's Ramon? I suppose he's out selling my clothes ... You know, I love dancing but I can't walk straight ..." Finally, at 6pm, he has a scene. The presidential suit is on, the presidential hair is in place. "Gad," he says, "I look like Richard Nixon." We swirl through the Biltmore to where the cameras are waiting to film his speech at a gala dinner. People bear down on him - ladies in pink jackets, suits in for a convention - all shaking his hand as if he actually is the President. An assistant director walks backwards in front of us, guiding us towards the ballroom. A beard and sneakers approaches. "This is the director," Sheen says. "He knows I'm a windbag. What am I saying?"

"You've got to say, 'God Bless America.'"

"That's very pedestrian," Sheen says. "I'll say a poem instead." The director laughs so much that his fillings show. Now Sheen is fully charged, for he has an audience of extras. He plays to them like a stand-up comedian, quoting W.H. Auden, mouthing the lyrics to Midnight Train to Georgia to see if they can guess what it is, cracking jokes about the Republicans, entertaining himself and everyone else.

After seven or so takes, the extras smiling and clapping, we edge back towards the lifts. People are still coming at him from all sides: assistants, suits, people with mikes. Everyone wants a piece. A German student approaches with love in her eye and a pen in her hand: "I've seen your show about 1,000 times ..."

"You're a lunatic," he laughs. "You godda gedda life."