Michael Sheen carries off the life of Kenneth Williams

To play the tortured comedian Kenneth Williams, Michael Sheen had to go beyond the flared nostrils and the cries of 'Matron', he tells James Rampton

I'm huddled behind a film camera in the corner of the front room in a desperately dreary Twickenham flat. It's a scene straight out of the 1950s - all drab flock wallpaper and austere post-War furnishings.

Here, Michael Sheen, the star of BBC4's new biopic, Kenneth Williams: Fantabulosa, is acting out one of the late comedian's sexual fantasies. He is snuggled up on the sofa next to a strapping Irish guardsman in full military regalia.

The soldier (played by Edward MacLiam) starts caressing Williams's thigh with one hand and his neck with the other. The comedian's ecstatic reverie is shattered, however, when he catches sight of the soldier's fingernails. "When did you last scrub your nails?" he shrieks. "Such dirt - positively filthy! I think you should leave."

The scene gets to the heart of Williams's misery: even in his dreams, he is unable to find fulfilment. Riddled with neuroses about being outed and obsessed to the point of mania with cleanliness, he is his own worst enemy. The film's message is clear: Kenneth Williams's biggest barrier to happiness is Kenneth Williams.

Over lunch, Sheen is in his caravan perusing a well-thumbed copy of The Diaries of Kenneth Williams. The actor looks worryingly thin. Since his last job - as the hale and hearty Tony Blair in The Queen, Stephen Frears's follow-up to The Deal - he has lost an astonishing two-and-a-half stone. "I did the cabbage soup diet." Sheen winces at the memory. "It was very harsh, but if you're motivated by a role like this, it makes it a lot easier."

For all that, Sheen is clearly having the time of his life playing Williams. A veteran of playing real-life characters, Sheen has steeped himself in research for the part. He has read dozens of books and seen hundreds of hours of archive footage. "I could perform ITV's An Audience with Kenneth Williams for you word for word. I've watched it hundreds of times," the actor says.

"It feels like I'm living with Kenneth. Someone like that takes you over. He's so obsessive, so full-on, so fascinating and so complicated, when you're trying to get to grips with him, you have to surround yourself with him completely. Like him, I've become obsessive."

The background work has paid off. Sheen has nailed the comedian's distinctive physical mannerisms - the flared nostrils, the pursed lips and the haughtily-raised chin. More than anything else, he has captured the chameleon quality of Williams's voice, an instrument that can go from one end of the Monopoly board - Old Kent Road - to the other - Mayfair - in one sentence.

It is also one of the most suggestive voices in recorded history. Sheen's Williams even manages to inject double entendre into the apparently innocent task of ordering lunch: "I'll have your melon balls, followed by the creamed chicken," he says, licking his lips lasciviously at the waiter.

Sheen, 37, an award-winning theatre actor who has a seven-year-old daughter, Lily, goes on to address the most controversial aspect of Fantabulosa: the issue of whether or not Williams committed suicide on 14 April 1988. The drama has grabbed headlines for suggesting that Williams was so depressed he took his own life with an overdose of barbiturates when he was just 62.

He was certainly feeling oppressed by Lou, his beloved yet increasingly demanding and senile mother (played in the drama by Cheryl Campbell), who lived in the next-door flat. Williams was also being tormented by seriously inflamed ulcers.

In the film, Williams reads out the final entry from his fabled diary: "Had meal with Lou at 5.30. Saw the TV news, watched the dreary saga of murder and mayhem. By 6.30 pain in the back was pulsating as it's never done before... so this, plus the stomach trouble combines to torture me - oh - what's the bloody point?"

Treading with tact, Sheen says, "One has to be sensitive about this because there are Kenneth's friends and family to consider. Understandably, the people closest to him didn't want to think that he committed suicide. But personally I think he did. He talked about it constantly and knew how to do it. By the end, he was in so much pain, he had nowhere left to go."

The public, though, never saw this dark side; Williams was one of the most popular entertainers of his generation. He built up a huge following through 22 Carry On films, Hancock's Half Hour, Round the Horne, Just a Minute and "the chat show years".

So why was Williams unable to take comfort from the public adulation? Sheen reveals: "There is this haunting entry in his diary, where Kenneth writes: 'How can anyone know how much I wanted love? But I was afraid of it, so I laughed at it or was cruel or spurned it.' That's terribly sad, isn't it? It's that plea for love that we're dramatising in Fantabulosa." (The title comes from the word that Williams would use in rare moments of joy - usually when in the spotlight).

Williams's great tragedy was that he remained, as he viewed it, a moral coward. In the 1950s and early 1960s - when homosexuality was still against the law - he could not bring himself to come out. So he remained a totally miserable celibate.

"He felt a huge sense of guilt about his sexuality," Sheen says. "In addition, he had a fear of anything that would compromise his fastidious cleanliness. He once said, 'I've never entered into life.'"

Would it be fair to describe this as an archetypal "tears of a clown" story, then? Not entirely. "That image - of a spurned lover - is romantic, whereas Kenneth was much more bitter and complex," observes Sheen. "He resisted love, but wanted it more than anything else. He was trapped by his own personality."

Yet even in the world of performing, where he appeared so at ease, Williams was, in fact, fundamentally dissatisfied. For instance, he derided his best-loved work in the Carry On films as "crapola".

According to Ben Evans, the producer of Fantabulosa, "Kenneth was a terrific intellectual snob. He felt he should be a classical actor, but found himself endlessly playing in end-of-the-pier burlesque. That just added to his deep sense of self-loathing."

So Williams was a troubled soul whose life story is a gift for any dramatist. Following films about Peter Sellers, Tony Hancock, and Sid James, Fantabulosa underlines our continuing fascination with tortured comics.

"Writers are obviously drawn to these stories because they don't have to think up a plot," Evans reckons. "But for viewers this sort of drama also generates excitement because it is giving you the untold story, the private side to the public face.

"And Kenneth Williams is exemplary in this regard - he had such a larger-than-life public persona and such a introverted and dark secret life. That dichotomy within him is what makes for such good drama. He could make anyone laugh. Except himself."

'Kenneth Williams: Fantabulosa' is on BBC4 at 9pm on Monday 13 March