Award-winning TV double act Mitchell and Webb are entering dangerous territory by transferring to the big screen. They talk to Steve Pratt about how easy it is to lose your head

Comedy double act David Mitchell and Robert Webb have split up. Before fans of the Peep Show pair of funny men start fretting, it should be noted that the separation is only temporary.

Mitchell has taken the armchair in the room, leaving Webb to share the sofa with magician turned TV producer Andrew O'Connor.

All three are together again to talk about Magicians, which marks the feature film debut of Mitchell and Webb, who met as Cambridge students while performing in the annual Footlights pantomime, Cinderella.

They're on dangerous territory. British comedies in general and TV double acts in particular attempting to transfer from small to big screen don't have a good track record. Morecambe and Wise were no Laurel and Hardy in their limited film career, while Cannon and Ball barely rate a mention let alone a put-down for The Boys In Blue.

Magicians works better perhaps because Mitchell and Webb aren't exclusively a double act. They both do solo work and their success isn't inextricably tied to the each other despite working together so much.

Since the two-man show they took to the Edinburgh Festival after the Cambridge meeting, their work together has included the Sony award-winning Radio 4 sketch show, That Mitchell & Webb Sound, and the Golden Rose-winning sitcom, Peep Show.

Mitchell's solo appearances include the Jennifer Saunders comedy, Jam And Jerusalem, as well as panel games such as QI and Best Of The Worst. Webb's work has been in BBC3's Smoking Room, Ben Elton's BBC1 sitcom, Blessed, and he has bared all as a naturist in the film, Confetti.

Ironically, in Magicians they play best friends Harry and Karl, the greatest magic double act in the country. Their careers take a dive after Harry's wife loses her head, first over Karl by sleeping with him and then literally in a horrible accident in the act's guillotine stunt.

Doing a movie at this particular point in the Mitchell and Webb career wasn't part of the grand scheme of things. "We've never had such a sense of organisation or strategy," says Webb.

"We wanted to do a comedy in whatever form people would let us. It probably came much earlier than we thought we would do a film. It landed very fortuitously on our doormat," adds Mitchell

Like most double acts used to working alongside each other, they let each other have a say. One doesn't take charge.

They attended early readings as the script went through various drafts and changes. "The more readthroughs you did, the more you thought this is going to be heartbreaking..." says Mitchell. "...when they replace us with Owen Wilson and Ben Stiller," finishes off Webb.

Peep Show executive producer O'Connor is a disgraced magician, having been thrown out of the Magic Circle for allegedly giving away secrets, so he was a natural to direct the film.

He and the film's magic consultant, Scott Penrose, were able to help Mitchell and Webb with the magic. They recall receiving a sort of welcome pack of DVDs telling the history of magic.

That didn't help Webb master the trick of rolling a 50 pence coin along his fingers in a smooth way. "I remember doing this is front of the telly for two weeks and not getting it right."

He's more used to stripping for the cameras after his full frontal exposure in Confetti. So he took his nude scene in Magicians, where a naked Karl is locked out of a hotel room, in his stride.

Him stripping on screen has been written into British law now, he jokes. "I thought 'here we go again' when I read the scene where Karl gets locked out of his room. But because I overdid it in Confetti, I shouldn't refuse to do things when they're funny. At least Karl does what normal people do, he covers his balls up with his hands."

Mitchell gets away lightly for it's Webb who also had to be buried up to his neck in sand in another scene. He felt very vulnerable with only his head visible on the beach, especially as it was late August and there were a lot of wasps about.

"As only your head is sticking out, you can't defend yourself if they settle on you. But I was well looked after, there was a hole in the sand with a crate and a stool for me to sit on. It wasn't terribly comfortable, I was getting a bit grumpy by the end of the afternoon."

I imagine O'Connor's directorial instruction: "Can you be funny just with your head" wasn't warmly received.

His head had its own star spot. He found putting it in the guillotine was still unnerving even after being shown all the safety mechanisms. It was, after all, his neck on the block.

"It half went through my mind it would be an awful irony if someone's head was chopped off," says Mitchell, who was operating the string controlling the blade. "I was thinking 'if I chop his head off, I'll never hear the last of it'."

Magicians has the advantage of following the movie success of another TV comedian, Simon Pegg, with Shaun Of The Dead and Hot Fuzz. "We make so few successful British films there's a lot of pressure on each one," says Webb.

"Every successful funny British comedy must help more to happen," Mitchell feels. "If more British films happened, people would get scared less and we could have more confidence."

The main challenge of the move from TV to cinema was coming up with a story that works over an hour-and-a-half. There's also a different dynamic between the pair than in Peep Show. Hopefully, says Mitchell, it's not as sour as the TV relationship.

"What's changed in comedy in the last 20 or 30 years is that comedians are very closely associated with their material," he says. "In the past, people could have a go at other people's material. Now, if you hear Ricky Gervais, you wouldn't add the line and try to get a laugh with it. Material is a lot more respected."

As if to emphasise they're not a double act, they don't share so many scenes in Magicians because of the nature of the story. "We've worked so much together in the 12 months running up to this film," says Mitchell.

"What's great is that the scenes we have together are a big deal in the context of the film. It was just the way the storylines developed. Both of us were in almost every day."

Magicians (15) opens in cinemas on Friday, the same day the current run of Peep Show ends on C4.