Al Jean, with his tweed jacket and curly grey hair, could be mistaken for a genial Harvard professor, which he nearly was except that he graduated instead from writing squibs for the Harvard Lampoon to The Tonight Show and his own prime-time animated show, The Critic.

Having dithered for years about translating The Simpsons into a full-blown movie . . . the series originated in the mid-1980s merely as one-minute animated asides commissioned by producer James L Brooks to break up The Tracy Ullman Show . . . Groening and Jean have now taken the plunge. After four years of intense secrecy, a final cut of the eagerly-awaited epic was completed three weeks ago. Amid high security Groening and Jean were in London to screen edited highlights to carefully vetted critics and media.

Whatever about the full movie . . .it would be foolish to reach a judgement on the basis of a mere 10 minutes . . . what is immediately clear is that the world's most loved dysfunctional family is in no way diminished by the transition to a big screen, as many had feared.

Far from being dwarfed when taken from the drawing room to the multiplexes, the characters revel in the freedom it allows to give their larger-than-life tribulations fuller rein.

Paradoxically, the larger they are the more human and immediate they become.

Having worked so well as short television episodes, it remains to be seen whether the sitcom format . . . which The Simpsons essentially is . . . can sustain a full-length feature.

A clip in which the US punk band Green Day are stoned by indignant Springfield fans when they try to sing about global warming . . .causing their floating stage to sink into the lakes . . . suggests that environmental issues will be juicy satirical targets, as will religion.

There's a hilarious Sunday church revivalist scene where Grandpa Simpson has a fit that has him speaking in tongues, causing the congregation to panic. Not knowing what to do, Bart thumbs through the Bible then tosses it away. "This book doesn't have any answers, " he complains.

The Simpsons have been around so long and is watched by so many millions around the world that it's no longer cool to be a fan. Selfstyled connoisseurs have suggested that it's lost its edge and become too safe. The Simpsons Movie should put that fear to rest.

While The Simpsons is almost unique in US television in having a noninterference clause . . . when Fox network was in the middle of a financial crisis when the show began, and was in no position to dictate terms . . .obviously tighter limits apply on television than in movies.

Groening and Jean seem intent on exploiting this to the full.

They've even come up with probably the funniest full frontal nude scene ever seen in mainstream cinema. Bart eagerly takes up Homer's dare to skateboard across Springfield to the Krusty Burger, stark naked. Every single fortuitous cover-up cliche is successfully employed to obscure his exposed member until at the last moment. As he nips behind a hedge with only his head visible, his arm catches in the foliage ripping open a slit that provides a shot of his disembodied penis zipping across the wide-screen like an icon on graph.

The Simpsons Movie pushes the boundaries of explicit violence as well as sex. Bart ends up with a hammer in the eye.

"That's our homage to Bunuel's Un Chien Andalou, " admits Groening.

"We thought about what we could put in the movie that we couldn't in television. In the US there's always someone willing to be offended by anything."

So what's The Simpsons Movie about? "It's that a man should listen to his wife, " says Groening.

"And it's a romantic movie, " Jean adds. "Homer falls in love with a pig." And the church scene? "We posit the existence of an extremely early Disney animation. It's a style Groening arrived at by accident. When he first pitched the idea of the characters to Brooks he thought he was just providing rough sketches for the animators to develop. Instead they used them as they were. "It's what I like about The Simpsons, " he says.

"It's imperfect. It's about the art of hand-drawn animation. It's not computer-generated where you have 1,000 perfect penguins."

The Simpsons television show, yet another season of which starts in the US this autumn, is a production line involving over 200 writers who work as a team, pitching ideas and rewriting each other's work over and over. Their brief requires that the characters never age or change and that every episode is a complete story. "After 400 episodes . . . one for every day of the year, Homer would say . . . some of the writers are so young that they grew up watching the show, " says Groening. "They've memorised every episode, so it helps prevent us repeating ourselves." "And if we do, " jokes Jean, "it's a running gag."

While working on the movie, Jean also continued running the show. "It was kind of awkward, " he says. "Writers would pitch a joke. No, I'd say, we can't do that. Why? You'll see on 27 July when the movie comes out."

It's a dilemma that cuts both ways. Lots of jokes had to be dropped from the movie during editing. "My favourite was a scene in which Bart gets trapped inside an Egyptian sarcophagus, " says Groening, ruefully. "When Marge berates Homer for letting it happen, he replies: 'He's got to get over his fear of coffins.'" He shrugs. "But the line didn't make it in the final cut."

Jean chuckles. "But it's going to be in the show, " he says. "I put it in. It was going to be a surprise for your birthday."

The Simpsons' characters are very personal for Groening. He set up the original idea in such a hurry that he named all the characters after members of his own family, partly to ensure that he wouldn't lose the rights to them. His father Homer was a cartoonist and writer. Marge is the name of his mother, who used to be a teacher.

"When I was small, she had tall hair, but not blue, " he says. The family lived in Springfield, Oregon.

"It was nothing like The Simpsons, " he claims. But then he would, wouldn't he.

Groening moved to Los Angeles in 1977 when he was 23. He described his early experiences there in a comic strip, Life Is Hell, which was picked up by the Los Angeles Reader where he also had a weekly rock column. "I remember giving a bad review to Danny Elfman's first band. It enraged him so much he wrote a letter to the editor. When it came to do The Simpsons and we wanted him to write the theme music, I thought he'd never remember it. But he did. Happily he forgave me."

Elfman's strident "da da da-da da, da da da-da" tune has become one of the most instantly recognisable in pop culture. "We gave him ideas for it but he came up with something completely different, " says Groening.

Groening branched out in 1998 with Futurama, an animated series about life in the year 3000 which ran on Fox for five years and is now being developed as four direct-to-DVD movies. So how much time does he still have for The Simpsons? "Here's the deal, " he says. "I tell the people at Futurama I'm working on The Simpsons and I tell the people at The Simpsons I'm working on Futurama. And then I go home."

Even Rupert Murdoch doesn't seem to mind that The Simpsons mocks the fundamentalist values that the company's sister channel Fox News propagates. He even appeared in an episode as a character called 'the evil tyrant Rupert Murdoch'. "He played it with relish, " says Groening. "But he wasn't pleased when he arrived late for a recording and someone said, 'About time you got here.'" The Simpsons cuts through all barriers because no matter how outrageously the characters behave, there's a sense that they're basically good people. "They're very misguided, " says Groening.

"Homer strangles his son, but he loves him. We're dealing with an elastic reality. We stretch it and then stretch it back. We obey the laws of gravity."

Al Jean agrees. "Homer is not intentionally mean, " he says. "He does thoughtless things. But Marge thinks he's the most wonderful man she's ever met."

'The Simpsons Movie' opens nationwide on Wednesday