Kenneth Williams: Michael Sheen carries on his camping

Kenneth Williams's extraordinary voice, outrageous double entendres and nostril-widening mannerisms made him an amazing one-off among British comedians. Now a bravura performance by Michael Sheen in a BBC drama recreates the often anguished public and private life. Hermione Eyre met him on the set

Night time, at Ealing Studios. On a back lot the premiere of Carry On Up the Khyber is being recreated. Here is the red carpet, the cheering crowds, the pressmen with their carbon flashbulbs. A Bentley rolls up and out steps Kenneth Williams, nostrils blazing, black tie immaculate, the skin stretched over his cheekbones in a rictus of delight. He gallantly takes the arm of his date for the evening, his mother Louie, and then sums up the crowd, the lights, the whole ecstasy of attention with one strangulated, orgasmic word: "Fantabulosa!"

"Cut!" calls the director Andy de Emmony. The cheering crowds revert to being chilly extras muttering about overtime, and the "pressmen" put down their props and go in search of location catering. Louie Williams becomes the actress Cheryl Campbell. And Kenneth Williams disappears gradually, like the Cheshire Cat, as the rigidity in his backbone relents, his nostrils relax and his vowels are repatriated. Soon just the actor Michael Sheen remains, saying gently in his native Welsh accent "Shall we get into the car for warmth, then?"

Inside the Bentley, the actors take turns at sharing a hot water bottle (rather generously they also offer it to me). Cheryl Campbell has a sore tooth, so she holds her mouth with a hankie while Michael Sheen takes her mind off it by recounting a story about the time he took his father as his date to the premiere of the film Bright Young Things (in which Sheen starred as Miles Malpractice). "The crowd went wild when my father got out of the car. Screaming, cheering, wanting his autograph. We couldn't work out what was going on until the penny dropped. You see, my father works part-time as a Jack Nicholson impersonator..."

Cheryl Campbell gurgles appreciatively. A runner pops his head into the car, collects the hot water bottle and gives them notice for the next take; the illusion re-assembles itself, and again out of the car springs Kenneth Williams.

It is currently a good time to be a great British post-war institution with an interesting private life. Anyone who answers to that description is now likely to get a BBC TV biopic made about them. In a period of about a year we have had Kenneth Tynan: In Praise of Hardcore; Elizabeth David: a Life in Recipes; Not Only But Always (a biopic of Peter Cook). A life of Fanny Craddock is also in the works at BBC4. The time is therefore ripe (you can almost hear him saying it - "Rrrripe, my deahs!") for Fantabulosa, the film of the life of Kenneth Williams.

He left excellent biopic material behind him when he died by his own hand in 1988 at the age of 62: a great public legacy of catchphrases ("Stop messin' about!"; "Hello I'm Jules and this is my friend Sandy. We're Bona Artistes...") and high campery, spanning Round the Horne, Hancock's Half Hour, and 24 Carry On films; and a great private ledger of grief in the form of his daily diary, which he kept for 43 years. Splice the two together and you have, as Ben Evans, the producer of Fantabulosa, told me, "humour and tragedy, pathos and bathos - it's all there." It also has one hell of a central performance.

Portraying the whinnying, goggle-eyed Williams is somewhere between heaven and hell for an actor, and the producers considered candidates including Chris Barrie, the Red Dwarf comedian; David Benson, an actor who has toured the country with his one-man tribute show Think No Evil of Us: My Life with Kenneth Williams; Robin Sebastian, who got Williams spot-on in the recent West End hit Round the Horne Revisited; and Little Britain's David Walliams, a Williams devotee.

In the end, though, Sheen won through because he was thought to be the most adept at physically inhabiting the role without reducing it to an impression. He has, after all, the background for it: he played Tony Blair in The Deal in 2003, and has since revived his Blair for Stephen Frears' new film The Queen, now in post-production; he is set to portray Dylan Thomas in a film about the poet and his wife, also forthcoming; and let us not forget that this chameleon ability also runs in Michael Sheen's family.

First, can he do the voice? That trademark "wasp with adenoids"; the voice that, as Elaine Stritch remarked when she appeared with him on Just a Minute, "can turn one word into a three-act play". "He tended to speak incredibly quickly, going from 0-60 out of nowhere. And he treated his lines like a musical score almost," says Michael Sheen. "It's a question of sort of hitting the right notes. And then you have to get it to veer between East End vowels and incredibly posh diction..." (This chameleon aspect of Williams's pronunciation has a parallel in his diary, which is written in many different types of handwriting, and can go from Copperplate to Roman to Italic in a single page. This could be simply due to his aptitude for calligraphy - he was apprenticed at 16 to a lithographer - but it is tempting to relate it to his many voices.)

Sitting in on the filming of Fantabulosa, I overheard Michael Sheen "revving up" his larynx before filming, closeted in his dressing room making little whines and hums like a toy aeroplane. Then, on set, his words came out at top speed, alternating wildly between Cockney nasal and fruity Received Pronunciation and exploding finally into a dirty, mirthless cackle. I recorded one of his takes on my Dicataphone - "Acting is a tightrope walk, and we tread it nightly...". Playing the tinny tape back, it sounds like archive footage of Williams himself.

And what of the physical side? Unlike Geoffrey Rush, who made a too too solid Peter Sellers in the recent biopic, Sheen is blessed with a skinny, boyish physique that is already approximate to Williams'. (Jonathan Miller once suggested Williams play Peter Pan.) Sheen has been on the cabbage soup diet nevertheless - useful for getting into character in more ways than one. "It has given me terrible wind, obviously - putting me in mind of all the gastric problems Williams had".

Williams suffered over his lifetime from severe intestinal pain, drawing a graph to chart the "progress of the bum" and beginning each diary entry with a summary of the weather and his bowels. The diary's last sentence reads: "the 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?"

To allow Sheen's diet its full run, the semi-naked scenes (shots of him having what Williams termed "a traditional" or "a Barclays bank", plus a delightfully high-spirited scene in which he does the Hoovering sporting a tight new pair of pants) have been left till last in the Fantabulosa shooting schedule.

But the face is all there from the beginning. Pinned to the mirrors in the make-up trailer are photographs of Williams at different stages of his career, for reference. Michael Sheen's hair has been cropped and Brylcreemed into Williams' neat, conventional comb-over, and his filtrum (the groove running vertically between nose and mouth) has been shaded for emphasis - as have the nostrils. "We considered putting plastic tubes up each one, to flare them a bit more," says Helen Barrett, the make-up artist. "But we decided against it," says Michael Sheen quickly.

With Sheen's transformation accomplished, let us consider the project as a whole. The 80 minutes of Fantabulosa are being shot at breakneck speed over only 17 days. Seventeen days might produce 15 minutes of a feature film; you could film a really glossy commercial in 17 days. But the project's low-budget speediness has its advantages, Sheen heroically insists. "Obviously I'd like to be able to go through scenes again and again, but doing it fast keeps everything very buoyant and fresh and spontaneous," he says, during a rare pause in a day on which he has filmed six key scenes in Fantabulosa, as well as completed a TV interview about his characterisation. It also lends the production an old-fashioned, hand-made quality in keeping with the period it portrays. And Williams himself found when he was shooting the Carry On films "a single take would often suffice".

Fantabulosa has other marks of authenticity. Not only is its script by Martin Heresford entirely based in truth (yes, when Williams played Princess Angelica in his school play aged eight, a reviewer really did praise his "comical demeanour and mincing step") but it also features the Just a Minute master himself: Nicholas Parsons makes a stretching cameo appearance as Nicholas Parsons.

"I liked Kenny enormously," he told me while the make-up artist was busily making him up as himself. "When he was off-stage he was natural and relaxed, not at all like the public performer. We used to have long conversations about the derivations of words." It is to Parson's credit that he has remained fond of Williams given, for example, this entry about him: "Mon 5 Aug 1968: Had David Hatch on the phone asking me to do six programmes in his radio series Just a Minute - unfortunately it means working with that Parsons fellow, but I said yes, 'cos it will be a nice fill-in."

Williams never quite changes his mind about Parsons, describing him over the years as "idiot Parsons" and a "stupid great nit", though strictly, this seems to have more to do with the context of the Just a Minute game than personal animosity. Williams also scrupulously recorded the time when Parsons "kindly" gave him a lift to Highgate. In the Fantabulosa make-up trailer, Parsons also does Williams a kindness, by reminding us of his erudition.

"People said he should have been an Oxford don. He was immensely serious about intellectual subjects: spirituality, musicology, Rilke and Thomas Mann and Lytton Strachey." (This sort of talk would have gratified Williams, who wanted very much to be thought of in this way. A diary entry in 1949 reads: "A Miss Knight called on me after the show and said she thoroughly enjoyed it. Said many patrons referred to me as 'Danny Kaye-ish!' This after Jean-Louis Barrault! The end!!!") Parsons also recalls how cerebral topics would exercise Williams on Just a Minute. "We also used to direct the topics towards areas he was especially interested in, to really get him going."

And, presumably, to rile Williams all the more when he was interrupted. Soon Parsons transfers from the make-up trailer to the set, where the Just a Minute recording studio has been recreated, ready to film the scene in which Williams becomes hysterical when he is buzzed for deviation. The actors playing Wendy Richards and Derek Nimmo and are sitting in place, powdered and bewigged, and the actor playing Clement Freud is having his cheeks stuffed with cotton wool, to add to the verisimilitude of his bloodhound jowls. Parsons gives some last-minute advice - "contestants should have their fingers on the buzzers at all times" - and the game begins. Sheen is soon shrieking "Whooo buzzed me? Ooooh was it you, Clement? You horrible great nit. I haven't come all the way from Great Portland Street to be treated like this!"

Cooling off in his dressing room shortly afterwards, Sheen tells me he feels Williams' Just a Minute outbursts were mainly for the audience's benefit. "He was licensed to do that - people wanted him to do that, people liked it." But Sheen can spot when Williams does genuinely lose control. "Watching An Audience with Kenneth Williams, there is a moment when he gets to the punch line, and - he messes it up. He gets it wrong. And he does this little jerk with his neck - a small involuntary spasm. For someone so controlled in all their mannerisms this is significant. I've used it occasionally in scenes when he is caught off guard by something." And indeed, in the scene when Tony Hancock reproaches Williams for being a caricature, he can be seen bridling inwardly, with a telling jerk of the chin. It's Sheen's very economical way of conveying the vulnerability that lay beneath the camp carapace.

Today, Williams would probably be diagnosed as obsessive compulsive. His habit of covering his cooker with cling film and refusing anyone else the use of his lavatory are giveaways. But there are signs he was even more psychically fragmented. According to Sheen: "The diaries aren't straightforward at all. He writes about going to the Establishment Club, and says there was this really awful middle-aged woman heckling and laughing and being really banal. Unknown to Williams, this show was being recorded, and I've heard it and it's not a woman heckling - it's Williams himself, cackling and making gurning noises. The diary is not a record of what happened so much as perhaps an illusion. The preservation of the illusion was terribly important to him."

Williams had a great talent for self-punishment. If he had given up self-scrutiny and self-hatred and allowed himself a few more pleasant illusions he might have lived a happier life. Perhaps this is why he once quoted this line from W Somerset Maugham's play The Sacred Flame as germane to himself: "What do we any of us live for but our illusions? And what do we ask of others, but that we be allowed to keep them?" He had by then relented, presumably, from his early judgement that Maugham's dialogue is "convoluted and stupid". Williams was a harsh critic (on returning from Ring of Bright Water he wrote: "about as bad as you can get in the movies. Even the otter was amateur.") but never more harsh than when his criticism was directed towards himself.

The mind boggles, therefore, to think what he would have made of his forthcoming biopic. One can only hope he would have adopted his finest Polari and pronounced it "truly bona".

'Fantabulosa' will be broadcast, BBC4, Monday 13 March, 9pm, as part of an entire Kenneth Williams evening; Editions of 'Hancock's Half Hour', 'Round the Horne' and 'Just a Minute' are available on CD from www.bbcshop.com