There are no such things as marrows in the United States. Americans call them something else. This may seem an insignificant matter to you, but it became very vexing indeed to Nick Park when he was working on his new Wallace & Gromit film with Hollywood money. "You see, in the film, Gromit is growing his own prize marrow for a giant vegetable competition at Lady Toddington's manor house," says Park.

So what's the problem? "At one point Wallace says, 'How's your prize marrow?'" Park overaccentuates the pronunciation of the word "marrow" to show the mouth shape necessary. "We'd already animated that scene. We'd done all the work fitting the mouth shape, second by second, to the word 'mar-row'. But it turns out that marrow is called squash in America." Park mouths the word "squash" to show what a different mouth shape the word would require if you had to, as he and his animators did, set those shapes in Plasticine. Such is claymation, Park's chosen form of expression, a technique in which Plasticine is minutely changed from shot to shot to give the illusion of 3D movement. A reshoot could have taken days, perhaps even weeks.

It didn't matter. Jeffrey Katzenberg, CEO of DreamWorks Animation, the Hollywood studio that has bankrolled Wallace & Gromit's $80m (£44m) feature film debut, told Park, not unpleasantly, that there was a big marrow issue and it needed to be sorted out. It was to be one of several culture clashes between the diffident, Plasticine-sculpting Englishman and the plain-speaking American studio boss during the five years it took to make Wallace & Gromit: The Curse of the Were-Rabbit.

The film is the second release in the five-picture deal between DreamWorks and Aardman Animations. The first, Chicken Run (whose stars were also made of the putty stuff), took over $100m in the US alone, making it one of Britain's most successful films. Katzenberg was clearly delighted: he saw big money in Plasticine. But by then, years had passed since Wallace and Gromit had had their last adventure and Park was obviously getting withdrawal symptoms. Chickens had provided a temporary respite, but it was clear that working with the toothy northern nerd and mouthless dog was where Park's heart lay.

So Wallace and Gromit were released from their temperature-controlled store and their first feature-length film was born. And with it, one imagines, a slight unease at DreamWorks HQ. W&G has a very British sensibility about it - Wensleydale cheese, flat caps and, of course, marrows are likely to be lost on much of American market.

"If there is a culture clash," says Park, "it mainly comes from the fact that DreamWorks are looking out for the American audience. It's not so much the east and west coasts, but that big majority in the middle. They're always reminding us of the difficulty they have in understanding British accents or terminology or labels for things. So we had a big thing with marrows. And we didn't want to compromise."

But it wasn't just a matter of creative compromise, or of Park and co-director Steve Box staying true to Wallace's Lancastrian character. There were practical issues at stake. Park, Box and their team of 30 animators would have had to unplug Wallace's Plasticine mouth, fit another one with a different shape, re-sculpting the Plasticine several times. Given that one day's work, at best, yields around two seconds of film, this was not a small matter. So what did they do? "We eventually agreed to change it," says Park, "but only for the American audience, from 'marrow' to - " he pauses for effect "- 'melon'!" Why? "Because it's the only appropriate word we could find that would fit with the mouth shape for 'marrow'. Melon apparently works over there. So we have Wallace saying, 'How's your prize . . . melon?"

In a telling indication of the way things are going, Aardman's animators have been moved en masse to DreamWorks' Burbank studios to work on their next feature: an entirely computer-generated affair called Ratropolis, in which Park is not involved. Park has managed to stay in Bristol with his Plasticine and keep DreamWorks at arms' length. So how has he managed to continue working with Katzenberg, a man not noted for being backward in coming forward? "We got on quite well, actually. He's a nice guy. But in any case, the fact that a guy over in Hollywood has such faith in Wallace & Gromit and Aardman that he will support us is an incredible situation."

He was, he says, on the phone to Katzenberg pretty much every day during the film's five-year production. "And then he flies over every 10 weeks on his private jet. He lands at Filton [aerodrome near Bristol] and comes in for eight hours of viewing. He comes to script meetings. It's basically story stuff he gets involved in. He doesn't really get involved in technical things. But he often makes comments and we either ignore them . . . no, I don't mean that! We do listen. But some things we just can't do. He seems to respect that."

Not that it's always easy. "Jeffrey's also very good at coming in and making quite cutting comments. Being American, he just says everything with great belief and no self-doubt at all. There's no 'Well, erm, wouldn't it be better if . . . ' or 'Maybe not, that's just my opinion.' There's none of that. It's: 'This is the way. You should do this.' But I've learned that he doesn't expect me to do everything he says."

The film has an A-list voice cast including Helena Bonham Carter and Ralph Fiennes - but the idea for it came when Park and Bob Baker (who wrote Wallace & Gromit adventures The Wrong Trousers and A Close Shave) were in the more humble setting of their local pub.

"At first it was called Wallace & Gromit and the Veggieburglars, and it was about these rabbits who keep invading Wallace and Gromit's vegetable plot, and then we started to develop that into an idea of a great rabbit beast who comes to town. A were-rabbit as opposed to a werewolf, because it seemed to fit the Wallace and Gromit world." Park and Baker were inspired by the venerable cycle of Universal studio horror movies. "They're all filled with blood and guts and we thought that that could really suit Wallace and Gromit's world because it's absurd. It's about people locking up their vegetables rather than their children."

Park says that the reason he originally decided to express himself in Plasticine - a medium that requires such a meticulous temperament - rather than anything less arduous or more glamorous, came down to the simple question of economics. "Plasticine was available when I was a teenager and started doing animation. I wanted to be like Disney, trying to film with plastic cels, but it was all too expensive. I didn't have enough money to buy cels, at least not enough to make more than four-and-half seconds of animation. But Plasticine was around, user-friendly and available to the masses. It was great because all you needed was camera, an Anglepoise lamp and a table. And you would make whatever you like come out of a blob of Plasticine."

He was inspired by Peter Lord and David Sproxton, who themselves had been drawing cartoons and animating clay as a hobby. The pair managed to sell a 20-second cartoon featuring a deadpan superhero called Aardman to the BBC TV programme Vision On. This success led them to establish Aardman Animations in 1976, and to develop a claymation character called Morph, who appeared in the children's art series Take Hart.

"I used to see what Pete and Dave were doing with their early stuff on TV for Vision On, which was a great, inspiring programme," says Park. "In a way my ambitions weren't to do anything more than Morph, really. I just wanted to come up with ideas."

The 46-year-old Preston-born animator started making the first Wallace & Gromit film, A Grand Day Out, when he was a student at Sheffield City Polytechnic (now Sheffield Hallam University). After working with Sproxton and Lord for a couple of summers while he was studying at the National Film and Television School, Park was taken on at Aardman, and finished A Grand Day Out. He also worked on the short animated film Creature Comforts, in which an off-camera interviewer asks claymation animals what they think about living in a zoo. It won Park an Oscar for best animated short film, the first of three - the others were also for Wallace & Gromit adventures: The Wrong Trousers in 1993 and A Close Shave in 1995.

In one interview, Sproxton said that his ideal was to make something for the Magic Roundabout slot on BBC1, that buffer between children's programmes and the adult early evening news in the 1970s. Did Park have similar kidult aspirations? "I've never heard the phrase 'kidult' before! But I think I know what it means. I think I've always had a strong memory of what I loved as a child, the kinds of things I grew up with and the atmospheres in films I loved. The magic evoked by films in the music or the lighting and the stories." And you wanted to evoke those memories in your own work? "Very much so. I grew up with a lot of eastern European films on BBC2. The Singing Ringing Tree, The Tinderbox - I loved those. And all the animation from Canada. But I was also inspired by all sorts of films, especially thrillers, and a lot of Hitchcock; I saw The Birds as a child and it inspired me.

"As a teenager, I was split in about 10 different directions. Part of me wanted to be like Ray Harryhausen. Part of me wanted to be like Terry Gilliam, that really bizarre, surreal stuff. I was such a big fan of his. And I wanted to be like Bob Godfrey who did Roobarb and Custard. I loved the fuzziness."

If you were a kid now, would you be working in CGI rather than Plasticine? "I probably would want to have a go at it. I fundamentally like drawing cartoons really and that's gone into Plasticine form. But there's something I like about just making characters out of clay." Are you not tempted to make a live-action film with real actors? "Not really. I have been offered stuff, actually, but I don't really have much ambition to do live action, because I think I would be a bit out of my depth. Well, I wouldn't mind having a go, but I wouldn't like to try anything too big."

But he already has tried something very big. Park has spent five years making a big-budget, feature-length film, while other movie directors churn out at least one a year, rarely with the same loving attention to detail that he insists on. His art may be the cinematic equivalent of slow cooking, but his efforts have taken their toll. He looks very tired. "I'm sleeping all the time now," he says with the look of a man who should be lying in a darkened room with slices of giant cucumbers from Gromit's vegetable patch over his eyes. Are you happy with the film? "I think so. I think we've really hung on to that small, quirky Britishness -" his tired eyes twinkle for a moment "- made with Hollywood money!"

A love affair played out in Plasticine

1976 Aardman Animations is founded in Bristol by Peter Lord and David Sproxton. The following year they create Morph for the BBC's children's programme Take Hart.

1985 Nick Park finishes his postgrad course at the National Film and Television School and joins Aardman.

1986 The company creates a groundbreaking pop video for Peter Gabriel's Sledgehammer. Park contributes the dancing chicken sequence.

1989 Britain gets its first sight of Park's Creature Comforts, one of five short animations commissioned from Aardman by Channel 4, and the first Wallace & Gromit film, A Grand Day Out, is shown at the British Short Film festival. It was Park's graduation project at the NFTS and cost £11,000. Lasting 23 minutes, it was made with characters built from Plasticine supported on metal frames. Creature Comforts and A Grand Day Out are nominated for best animated short at the following year's Oscars. Creature Comforts wins.

1990 Park's series of commercials for Heat Electric airs on British television. Closely based on Creature Comforts, they use the same technique of fitting vox pops of real people to animated characters. The campaign is a great success but is often remembered, wrongly, as being for British Gas.

1993 The Wrong Trousers, Park's second Wallace & Gromit film, is the highest-rated programme on the BBC's Christmas schedule. At 30 minutes, and with a budget of £650,000, it is Aardman's most ambitious project to date, complete with dramatic lighting and Hitchcockian mise-en-scene. "I really wanted The Wrong Trousers to have the look of a mini feature film," says Park. It wins an Oscar.

1995 A Close Shave is another huge Christmas success. Although it is still only half-an-hour long, the film requires a much larger production team in order to be finished on deadline. The budget this time is £1.3m. It wins another Oscar.

2000 Chicken Run is released in June, the first of Park's five-picture deal with DreamWorks. Based on The Great Escape, the film is 84 minutes long, costs an estimated $42m and stars Mel Gibson. It is a hit, but as it is not a short, it doesn't win an Oscar. It is credited, however, with bringing about the new Best Animated Feature category.

2003 In February, BBC3 launches on digital television, featuring a series of Creature Comforts-style animated "blobs", created by Aardman, as its channel ident.

Leo Benedictus

· Wallace & Gromit: The Curse of the Were-Rabbit is released on October 14.