He admits that when he was working on three shows (Roger, Roger, Playing the Field and The Fast Show) virtually simultaneously last year, "I started to have a personality disorder. I'd get a fast car from one set, have five hours' kip and then turn up at another. I'd think, `hang on, the crew have changed,' and then realise it was a different show." He has now dyed his hair a violent shade of blond as "a rebellion against continuity" - but he's no less in demand. During a recent two-week holiday in Mauritius, he missed out on "three series and two films".
But just why has Thomson become the busiest thing this side of a fly with a distinctly azure behind? It must be something to do with his versatility. Whether he's Paul Calf's friend the delinquent Fat Bob, the ghastly smoothie Jazz Club Man in The Fast Show, or the dorkish barman, Ken, in Men Behaving Badly, Thomson is always unshowily reliable. His characters invariably have a bloke-from-down-the-pub recognisability about them.
"I remember doing Chekhov at the Manchester Poly School of Theatre and thinking `aaargh, Chekhov'," Thomson recalls. "But the director said I didn't understand it because I wasn't thinking `oh, that character's like so-and-so in the canteen'. Chekhov could be set in Salford in 1998. People still have the same quandaries. Ignoring the fact that you're wearing a period suit and Schubert glasses, people identify with the naturalism of a scene - and that's what makes it work."
The other reason Thomson is popular with casting directors is that he doesn't want to hog the limelight. He's a solid middle-order accumulator rather than a flashy opening batsman. "I like being the sort of jobbing character actor who gets to play different parts," he says. "I wouldn't fancy my own show - all teeth, tinsel, dancers and singing `Love Is In the Air'. Anyway, after Emma Thompson's show, what could I call it - `Thomson II'?"
He certainly works best as a team player. Some of his finest work has emerged from his joint writing with Paul Whitehouse and Charlie Higson on The Fast Show - the Nouveau Footy Fan, for instance, or Jazz Club Man. The latter has prompted complete strangers to come up to him in the street and say "nice" - to which he replies "great". "I'm glad the catchphrase isn't `rubbish'," Thomson says. "That would make me paranoid."
His other great collaboration is with Steve Coogan, whom he first met when they were both studying drama at Manchester Poly. After sharing a Perrier Award with Coogan at the Edinburgh Festival, Thomson went on to play Pauline Calf's partner, Fat Bob, in both The Paul Calf Video Diaries and Three Weddings, Two Fights and a Funeral. But for all Bob's cultish popularity, Thomson is quite glad to have finally evaded his clutches. "I now go for less stoogey, fall-guy stuff. I did all that with Fat Bob. I was in danger of becoming rent-a-chump. I love that character, but I wouldn't want to spend the rest of my life being typecast as him - I'd rather be typecast as Doctor Who."
That's not to say that Thomson didn't benefit from working with Coogan. "Steve's genius is his total dedication to his characters and attention to detail," he says. "When we write, we'll take half an hour over one word. You've got to be specific. If we read a script which asks for a Lada, we go `oh no'. But if it says, `an Austin Maxi 1100 with one yellow door,' well - that kind of detail is what makes Steve's characters work so well."
For a while, Thomson and Coogan have been hoping to make a spy spoof called The Knight. "Steve would be a character called Knight Cheddar Blaze, who is an international chess grand master, but also a secret agent. I'd be his boss, M, but also a master of disguise. A real woman would approach him, and then the camera would cut to me in her frock."
In the meantime, Thomson would like to develop his career as a jazz drummer (I kid you not) and build up his portfolio of feature films, which currently includes The Young Poisoner's Handbook, The Man Who Knew Too Little and The Girl With Brains in Her Feet. He also acted in The English Patient. No, you weren't dozing during his scene - it was cut. "I played a Second World War soldier chatting up Kristin Scott Thomas at an open-air cinema in Cairo. At the time I had no idea what the film was going to become - I wasn't bothered that I'd been cut. Then, nine Oscars later, I was bothered."
Something of a one-man sketch-show, Thomson makes for invigorating company. As we make to leave the central London bar, he can't resist a Fast Show gag. Dashing to be first to the cloakroom, he calls back to me: "I'll get me coat."
`Playing the Field' starts on BBC1 later this month. `The Fast Show Live' video is available in the shops.