Making a movie about a novel said to be unfilmable didn't deter actor Steve Coogan, he tells Steve Pratt, even though it was like pushing a supermarket trolly with a wonky wheel.

Steve Coogan is recalling that being suspended in a giant latex womb was very uncomfortable. This was partly his own fault as he made it seem even more uncomfortable for the sake of the comedy. "It was pretty unpleasant. I mean, if you hang upside down all the blood goes to your head, which isn't nice," admits the man doomed to be forever identified with his comic creation, Alan Partridge.

Matters were made worse by a flush of after-birth - a mix of wallpaper paste and water - that soaks him in the womb. "It was very cold and I was nearly naked apart from a thong and they were throwing buckets of this stuff over me," he says. "So that was probably the most unpleasant aspect, being in the womb and covered in goo. But if it gets a laugh, I'll do it."

He was doing it for a surreal scene in the new film, A Cock And Bull Story. And now he's doing it again - making people laugh - with fellow funny man Rob Brydon, from TV's Marion And Geoff, to promote the movie. They're unlikely to become the next Morecambe and Wise, but their interview double act reflects the nature of the movie.

A Cock And Bull Story is a film of Laurence Sterne's literary work, The Life And Opinions Of Tristram Shandy, Gentleman. The book is generally reckoned to be unfilmable. That hasn't stopped director Michael Winterbottom trying - and generally succeeding - in producing an amusingly off-the-wall comedy about making a film about a film within a film being performed by actors playing fictional characters, themselves and fictional versions of themselves as the action switches back and forth between the 18th century and a group of 21st century film-makers.

As well as Coogan and Brydon, the cast includes X-Files star Gillian Anderson, Keeley Hawes, Dylan Moran, Kelly McDonald, Jeremy Northam, Shirley Henderson, James Fleet and Ian Hart.

The project has had a long and difficult history. Brydon was first approached by Winterbottom - with whom he'd worked on 24 Hour Party People in which Coogan played music promoter Tony Wilson - about appearing in a TV series based on the book.

"I hadn't heard of it, so I went to Waterstone's, looked at it, thought it was too big for me to read and forgot all about it," he says. "It was only about two weeks into filming this that the penny dropped that this was the project Michael told me about two to three years ago."

The film script proved equally baffling to Coogan, as it was only about 60 pages long and unfinished. "Michael has a strange way of going about films. He schedules it, then makes it whether the script is ready or not and says we'll worry about the script when it comes to filming," he says. "So I read this and, if it had been anyone else other than Winterbottom, I wouldn't have done it because it looked too self-indulgent. I thought at worst by doing this with Michael, it would be a noble failure rather than just a cliched film. I knew it would be original and different from anything else.

"He works largely on instinct and I trust his instinct. It worried me slightly because I thought it was a bit risky."

The director's playing-it-by-ear methods extended to the on-screen relationship between Coogan and Brydon as he adapted to their strengths and allowed improvisation. "He doesn't have a pre-ordained view of exactly how the film should be. It's a bit like a shopping trolley that's got a wonky wheel, so whenever you push, it goes off that way. Michael doesn't fight it, he goes that way. He just wants to get up a good head of steam," says Coogan.

He moves from one analogy to another, starting to liken the sparring with his co-star with a tennis match. "Oh, no, not the tennis," says Brydon sarcastically. Coogan continues anyway: ""Rob's like a good tennis partner. He raises your game. You know that you're both in tune with each other.

"The difference between a good comic actor and a comedian is that if you put a comedian in a film they will just do their shtick. But if you have any competence as a comic actor you can do the funny stuff but also listen to the other actors and respond to them. Sometimes I'll pick up and be led by Rob, and vice versa. It's really fruitful, stimulating and enjoyable."

He's beginning to sound worryingly serious. So does Brydon, taking up the topic to explain that aspects of his and Coogan's real life relationship are reflected in their on-screen characters. "Over the years it's been a very warm relationship but also a bit spikey," he says. "We've had our ups and downs, and there is a competitiveness but a healthy one. In the film, we make it a little less healthy."

Coogan, in particular, must be wondering if A Cock And Bull Story will be the movie to finally let him leave the Partridge family and go on find movie star fame. So far, he's had modest screen success in The Parole Officer, 24 Hour Party People and Around The World In 80 Days. He's "a bit jaded" because he thinks every film might be the one that turns things around for him and "finally lay the ghost of Alan Partridge to rest", he says.

"The best that this film can do is that there's a possibility it could reach a slightly larger audience if word of mouth is good. But if you try to describe it to people they might think it sounds horribly self-important, turgid, avant garde and inaccessible. In actual fact, it's a lot lighter and more palatable than you'd think if you saw the synopsis."

He has mixed feelings about recognition in the US, although 24 Hour Party People was "noticed more by the cognisenti in America than it was here". The movie led to a role in the forthcoming film about Marie Antoinette from Lost In Translation director Sofia Coppola.

"But if you're asking whether I need affirmation by Americans to feel valid, then no, no really," says Coogan. "I'm currently writing a TV series because I don't see film as the be all and end all, or TV as the poor cousin. I love good television and want to do more of it.

"I like working in America as a different kind of experience but I'm currently about to start shooting a new TV series with a new character, and I'm more excited about that.

"I want to do different things, but what I also want to do for the people who like me and the comedy I've done, I want to honour that patronage by trying to do more of that funny stuff that they like. That audience has helped me so I feel an obligation.

"Unfortunately, I'm not that prolific. When you've done something that people like so much you have to make sure that the quality of what you do next is up there with it, otherwise you get hammered."

Brydon, last seen on TV in the Australian-set BBC2 comedy Supernova, is pleased, from a career point of view, it's great to have something more than three lines in a movie and make a bit of an impact. "I'm very pleased because I'm doing the sort of humour that I think of as my style of humour, and I'm very happy to see that on screen," he says.

"People always say, 'America, America' but I've never yet been there and not felt a bit weird about being there and away from home. I'm sure it would change if I had a wonderful offer but there isn't some great hunger in me for America."

* A Cock And Bull Story (15) opens in cinemas on Friday.