It’s murder trying to get The Fast Show people to be serious for a moment, so why bother? Wait long enough, until they’ve gone through their whole range of characters, from drunk toff Rowley to tragic jazzer Louis, and they just might go reflective all of a sudden. It’s a long shot, but worth a go.

Paul Whitehouse and Charlie Higson are the Lennon and McCartney of this disparate group of writers and performers. The team they headed came up with four TV series of increasing popularity between 1994 and 2000. They were sketch-based and as quick as the title suggested. They were relentlessly juvenile but also too perceptive to be written off as vacuous. And they were influential beyond their BBC Two ratings. The reason the members are now reassembling, five years after their Farewell Tour, is that they are doing another, last, live, West End reprise of their material at the Dominion Theatre. With a Pythonesque sort of self-parody, they have called this show Shamelessly Plugging the DVD.

“I could have done more touring,” says Whitehouse, “but I got the impression, perhaps erroneously, that Higgers here didn’t want to do any more live stuff. I suppose we’re like any collaborations – we’ve had our differences.”

“No we haven’t,” says Higson.

“Oh, funny guy,” says Whitehouse. “It’s very fashionable now, re-forming. Everyone re-forms and does tours. Bob Dylan has re-formed, although apparently he’s really ugly now. Oh yes, and we’ve got the radio show, Down the Line.”

TV to radio: that’s the reverse of the usual progress for comics. “Yes, we’ll be doing student revues next.”

This conversation is taking place in the tiny King’s Head Theatre pub in Islington, chosen because it is not far from their homes in North London. Whitehouse is more Highbury than Islington, although he supports Spurs, not Arsenal. Simon Day (Competitive Dad and Dave Angel, Eco-Warrior in The Fast Show), has just slouched off disconsolately, muttering something about his place in the pecking order.

No disrespect to the rest – which includes Arabella Weir, John Thomson, Caroline Aherne, Mark Williams and Paul Shearer – but by working with Whitehouse they had the mixed fortune of trading with a bona fide comic genius. Being now just a few feet from him, and seeing his face flick in an instant from Suit You tailor Ken to pub bore Archie is to be reminded how, like Rory Bremner, he represented a breakthrough in the technology of his craft; Johnny Depp became obsessed by the show and called him “the finest actor of all time”. Because his approach is so instinctive, so triumphantly untrained, to inquire about the process is to seem as ludicrous as Shaw’s Professor Higgins in Pygmalion, trying to pinpoint the origin of every vowel sound.

So here goes. Paul Whitehouse’s brilliance can be explained by a phenomenon known as the Silent Month. This occurred in 1962 in the town of Enfield (nothing to do with Harry).

He was four years old and had recently moved there with his parents from the Rhondda Valley in South Wales, where his father worked for the Coal Board. “At school I didn’t say a word,” he says, “I think because everyone was speaking so differently from how it had been in Wales. And then, after a month, I came home one day and said, ‘Muumm, I wanna go to Sarfend!’ For her that was the end, because I had lost my lovely Welsh lilt. So I became very conscious of speech, and the effects it can have. When we went back to Wales to visit, I would start talking all Welsh, lyke thaht, you see, and then, coming back the other way, like Alf Garnett. It was a matter of having to adapt, and adopt a voice which would fit in.”

When I do get to speak to Simon Day, he makes the point that The Fast Show was different to other TV comedy shows, particularly BBC Two ones, in having a strong working-class input. “Like Paul, I’d knocked about in all sorts of unglamorous jobs,” he says. “I’d been a builder’s labourer in Greenwich, worked for a printer in West London, and for the first McDonald’s to open in Britain. There were characters in the show that plumbers and builders could identify with.”

But then there were plenty more with whom the channel’s core viewing base would feel quite at home – Patrick Nice and his unusual experiences, such as finding the original copy of the Bible, “which was nice”, or crumpled old lecher the 13th Duke of Wybourne, always turning up in girls’ boarding schools or other inappropriate places. If one was going to assess The Fast Show in terms of its class appeal, then breadth was the key, for here was a gallery of gargoyles taken with equal conviction from every social stratum. None of the characters was patronised by clever graduates on the one hand, or crudely lampooned through envy and ignorance by University of Lifers on the other.

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