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If proof were ever needed that David Walliams has hit the comedy big time, it came not so long ago while he and Matt Lucas were on their 2006 Little Britain Live tour. “After one performance, Kate Moss and Pete Doherty came backstage, but so did Sir Paul McCartney,” Walliams says. “We had to keep Kate and Pete waiting while we were talking to Sir Paul. And I’m sorry, but I do still get excited about that sort of thing. There is still a big wow factor involved in my own success.”

The kind of success that he’s talking about includes the ability to fill stadiums to capacity, to name his price on an upcoming HBO version of Little Britain in the US, and to turn his hand, if he so chooses, to serious acting roles, like his recent performance in Stephen Poliakoff’s highbrow TV drama Capturing Mary. On the home front, too, he seems conspicuously to enjoy all the trappings of the high life. He lives in Noel Gallagher’s old house, Supernova Heights in Primrose Hill, and has dated a bevy of celebrity mag princesses. He drives a soft-top Mercedes sports car, and is a stalwart of the London party scene. A far cry, indeed, he admits, from the often down-at-heel and slightly tragic types that are his forte on Little Britain. For while in the show he plays carer Lou, all shell suit and dodgy dentistry, or Anne, the madwoman in a winceyette nightie, in real life you’re more likely to find him posing on the pages of Esquire in a Savile Row suit, hair slicked back, something that looks suspiciously like a sense of absurdity at his own situation playing behind his eyes and twitching at the corners of his mouth.

“There is part of me that does wonder, ‘How on earth did I get here?’” he says. The answer is through sheer determination. Walliams is not just a party animal; he has a reputation for strenuous, even obsessively hard work – whether creating sketches and characters for Little Britain or training, as he did in 2006, for a successful cross-Channel swim. His trainer back then, modern pentathlete Greg Whyte, says he has never encountered a similar level of dedication in anyone, including the Olympians he has trained. “An incredibly tenacious and single-minded individual, he didn’t miss a single training session in nine months.”

Not surprisingly, then, when I meet him today on the set of his latest project – a BBC Four drama in which he stars as the iconic comedian Frankie Howerd – he is displaying the same commitment all over again. Even when no one has called for action on today’s scene, a recreation of the famous speech given by Howerd at the Oxford Union in 1990, Walliams can be spotted running through an inventory of Howerd’s facial tics and physical mannerisms. He plants the palms of his hands on his lumbar region in the manner of Lurcio in Up Pompeii, raises his eyebrows and puffs out his cheeks in an “Ooh missus” gesture. By the time the camera rolls on the scene, the resemblance is uncanny.

Clearly, Walliams feels a deep connection to the role, not least, perhaps, because he can relate to some of the darker aspects of Howerd’s nature. As Frankie Howerd: Rather You Than Me demonstrates, the comedian suffered from depressive interludes all his adult life. Walliams, too, has fought his own demons. In the late Nineties, just as he and Lucas were enjoying their big professional break – they were about to film the first TV series of Little Britain – Walliams became exhausted and anxious, struggling to get to sleep, only to wake in a cold sweat with nightmares. He admitted himself to a psychiatric hospital.

“Looking back, I suppose I was just unhappy for personal reasons, and if you’re unhappy for a long period of time, you stop being unhappy and start being depressed. You get to the point where you just can’t see the future,” he explains. “And that was where I was. But plenty of people have suffered from depression, including Frankie Howerd himself.”

Howerd’s depression – both its origins and the controversial LSD therapy he received in the Sixties – form the cornerstone of the revealing BBC Four drama. From the same stable as 2006’s Kenneth Williams: Fantabulosa! and last year’s Fear of Fanny, about Fanny Craddock, it is one of a season of four new dramas that will examine the private woes of public icons of the Sixties and Seventies. The Curse of Steptoe has Phil Davis and Jason Isaacs playing Wilfrid Brambell and Harry H. Corbett, two actors who loathed each other but were yoked together by their success for all eternity. Hancock and Joan, starring Ken Stott and Maxine Peake, will tell the tale of Tony Hancock’s affair with Joan Le Mesurier. Hughie Green, Most Sincerely, with Trevor Eve, explores the double life of the avuncular host of the hugely successful TV shows Opportunity Knocks and Double Your Money.

Each presents a no-holds-barred insight into its subject. Howerd, for example, is depicted as a toupee-wearing, conflicted but predatory homosexual, given to crippling self-doubt. Not, then, the most flattering portrait. Yet, according to Walliams, the drama attempts to understand and explain rather than to expose. “Hopefully, we’re shedding some light on what made Frankie tick and on why, along with the great high points in his career, there was also at times so much vulnerability and pathos, and many low points.”

In comedy terms, Walliams, after all, sees himself as a direct descendant of funnymen like Frankie Howerd and Kenneth Williams. He owes them a debt of honour. “There is a long heritage of camp comedy in Britain – we’re a culture that has always loved pantomime dames, for example. Matt and I are from the same tradition of camp British comedians; we have always found that kind of thing funny.

“If anything, Frankie played down his gayness. In fact, if you had to pick one image of him it would probably be as Lurcio in Up Pompeii, surrounded by all these busty women. Preparing to play him, I was watching tapes of the Oxford Union show that we’re filming here today, and he talks about having an affair with an older woman. And part of you thinks, ‘Ooh no. I don’t think so.’ But the other part sort of believes him. He could almost pass, you know.”

Walliams, who on screen is often easily camp enough to pass as gay, could almost be accused of playing down his straightness, his sexuality having been the subject of a “gay-o-meter” in The Sun. Snapped at Nobu with a beautiful starlet? The gay-o-meter flips to solidly straight. Seen shopping in Prada in Bond Street? He’s back in the pink. “When they find out that I get fellated by Rafe Spall in this, the gay-o-meter will go fuchsia,” he jokes, referring to his co-star in the production. “I also get to share a bath with him and we touch each other’s penises, which was a thrill for us both. Although afterwards there was a lot of talking about girls and studying Nuts and Zoo magazines. A lot of reasserting our heterosexuality.”

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