Comedy: Little big man Yesterday Slough, tomorrow the world. The Office fall guy has landed on his feet, says Stephen Armstrong As you settle down in front of the telly to watch the Office Christmas special, or possibly crank up the DVD to watch series two, see if you can guess the answer to this little seasonal brain-teaser: which member of the Office team is now a big-league Hollywood player, appearing in multimillion-dollar movies with global megastars and lounging around in vast Winnebagos with kitchens and double beds? Ricky Gervais, the star and writer? No. Martin Freeman, the romantic lead? Guess again. Mackenzie Crook, the skinny stand-up comic who plays the fall guy, Gareth? Could be. After a role in Jerry Bruckheimer's summer megaflick Pirates of the Caribbean, Crook will be reunited next year with Johnny Depp, in Neverland, and Geoffrey Rush, in a biopic of Peter Sellers. He's also sharing the screen with Christian Slater and Neve Campbell in Churchill: The Hollywood Years; with Christina Ricci in The Gathering; and with Heath Ledger in The Brothers Grimm. And he co- stars with Johnny Vegas a dream casting, if ever there was one in Sex Lives of the Potato Men. Next year, Crook will be all over your cinema screen like a rash.

When I first meet him to discuss his rise from small-screen girner to big-screen earner, he's still a little fazed after attending the previous night's London premiere of Pirates of the Caribbean. He'd been out at the front of the Odeon Leicester Square, pressing the flesh with Orlando Bloom, Keira Knightley and Jack Davenport who had tried to persuade him to do a Mussolini salute from the cinema balcony and he doesn't seem to have quite recovered.

"I've been to awards ceremonies before, but this was crazy, crazy." He shakes his head, sitting up smartly in the chair in the hotel suite his PR has booked for interviews. "This time, people were calling out to me, wanting to talk to me. I had to come up with impromptu witty replies. And sign autographs. I've done it before, for blokes in the pub who like The Office, but this crowd were weird. There was a bunch of professional autograph-hunters here who don't have books, they have index cards, and they sort of file them away."

He looks so bewildered and vulnerable sitting there, baffled by the world he's wandered into, that you almost want to hug him. In part, it's his frame. TV is said to put 10lb on you, and Crook already looks painfully thin on camera. In the flesh, and there isn't a great deal of it, the first impression he gives is one of frailty. His soft voice and serious mien, and the lengthy deliberation he gives to answers, increase this sensation.

To be fair to the man, this impression can't be entirely accurate. Crook has been a singer in a heavy-metal band albeit briefly and spent years trawling the comedy circuit as a variety of comic characters, most recently the bumbling teacher Mr Bagshaw. Comedy- club audiences are not renowned for their supportive, giving attitude, so Crook has to have balls of steel beneath his jeans, right?

"Well, people say how shy I am, but I don't really believe it," he says thoughtfully. "I think I'm probably quite confident. You have to be stand-up audiences can be vicious. Mr Bagshaw works nicely in the clubs, because it's really clear what the audience's role is, as pupils in his class. It's funny, encouraging people to behave like kids in a class again; it's almost like therapy for some of them. It's fun, but the stand-up was always a means to an end. I was using it to get ahead as an actor, because I had no formal training."

Crook came to the game late, having originally planned a career in graphic design, until he failed to get into art college. He grew up in the commuter-belt town of Dartford, Kent, attending the same all- boys grammar school that educated Keith Richards. I mentally noted his build and his voice, remembered my own skinny frame at my old school and assumed that he was bullied.

He wasn't. "I didn't have an especially tough time," he says distantly. "But it was a frustrating school. There was no drama department, and some of the teachers were evil. I couldn't wait to leave. That's probably why I couldn't face drama school I couldn't bear going back into a classroom. Still, it worked out okay. I've got this theory about suburban towns like Dartford. They can inspire great creativity, like the Rolling Stones, because they are so dull, it encourages people to get out."

So he rebelled, as suburban kids often do. Dyed his hair. Joined a metal band "I was lead singer, front man. It was the early 1990s. I had really long hair, but I've got quite a girlie voice. I was trying desperately to be Kurt Cobain, but it wasn't happening." And wound up doing comedy, living in a grotty flat in Hounslow, eking out an existence on low-paying stand-up slots.

"I know my parents were worried for years when I was doing stand-up," he admits. "Poor as a church mouse, eating beans on toast. They never tried to dissuade me, though. I took my sisters to the premiere party last night. They were stunned by the whole circus." The stardom arrived at the right time. In January, Crook had a son, Jude, so the money comes in handy. It has enabled him to buy a flat in north London, some Thomas Hardy first editions and a signed photo of Kurt Cobain. He met Jude's mother, who works in advertising, when she was running a Soho comedy club. Unfortunately, their 2001 wedding coincided with the filming The Office, so he was sporting Gareth's nerdy haircut. Not that he's complaining.

"The Office made me," he says simply. "The director of Pirates spotted me on the plane when The Office came on the in-flight telly. All the film roles I've had have been from The Office. Of course, it means I'm playing the comedy skinny guy in fact, my roles have been drug addicts, drug-dealers, weirdos, perverts, pirates and ghosts. But I'm fine with that."

And so, in an era of Bloom, Knightley and Grant, it's time to welcome the newest British star in Hollywood's firmament one who's uncomfortable if his Winnebago is too big, and who hates jumping queues. One who returns calls and is always eager to please. Indeed, with his self-effacing humour and gentle, slightly awkward manner, Mackenzie Crook could be the most British star of all.

The Office Christmas Specials are on BBC1: Boxing Day at 10.15pm and December 27 at 9.50pm