It's the film the fans have been waiting for, for an awfully long time. Two decades after The Simpsons made their TV debut, the eagerly anticipated movie is out next week. Steve Pratt has a sneak preview.

TWENTY years ago, Matt Groening was asked to provide animated segments for the US comedy series, The Tracy Ullman Show. Unwilling to give up his rights for his popular Life In Hell cartoons, he sat down and created a new family and called them The Simpsons. The rest, as they say, is history.

Now he finds himself, with producer and writer Al Jean, sitting in a London cinema among an audience watching exclusive footage from the eagerly-anticipated, top secret The Simpsons Movie.

The film finally reaches cinemas next week after being kept under wraps by makers Fox until shortly before release. Watching the reaction of Simpsons' enthusiasts to the ten-minute taster is enough to convince you that the makers have got it right.

Both Groening and Jean are well aware of the expectations fans have of the movie, which they began contemplating in 1992. Equally, they know the pitfalls of extending a half-hour series into a 90-minute feature film.

"What's interesting about the experience of doing the movie is we work on the show with each other and then we go home," says Groening. "So this experience of coming out and meeting the people who love what we do is different. This is the first time a real audience has seen the excerpt from the movie and the response from sitting back there was very gratifying."

So what can we expect as Springfield's most famous family - Homer, Marge, Bart and the rest - debut on the big screen? It appears that Homer must save the world from destruction. Issues like the environment and religion come to the forefront. As does Bart, skateboarding naked down the main street of Springfield.

One reason the film has taken so long to happen is that the TV show is a year-round occupation. "We've always been working on the show," explains Groening. "We do 24 episodes a year and it takes six to eight months to do a single episode. You can add that up. It's a lot of work.

"We don't have time, we don't have a team waiting in the wings ready to do a movie. And finally, finally, we said we're coming up to our 20th year of existence and our 400th episode, we should have a movie out."

With so much riding on the film, Jean reckons: "We argued in a constructive way over every minute of the film, trying to make sure things were right on story, they were funny and in character. It really was something we couldn't have felt more pressure on, just because of the love people around the world have had for the show over the years."

Practically every character from the series features in the movie. "We thought about what we wanted to put in the movie that we couldn't show on television, like Bart naked," says Groening. "Also, we argued over jokes, every single scene received an incredible amount of attention and, because we didn't have a strict deadline to begin with, it took a long time to write the script."

But what exactly is The Simpsons Movie about? "I'd say in one sentence it's that a man should listen to his wife," suggests Jean. "And it's a romantic movie. Homer falls in love with a pig," adds Groening.

What he calls "the Simpsons all-star team" were responsible for the movie. "It was some of the best writers who've ever written for the show all together in a very small room, all sitting round eating cold pizza until the wee hours."

Fox, not unnaturally, wants to attract as wide an audience as possible, which means they need non-Simpsons' devotees to hand over their money at the box office. "Our mission was if you'd never heard of The Simpsons, you could enoy the film," says Jean.

"It's important it's viewed as a separate entity. It's not something where you have to see the show to find out the end of the movie. If you pay your money, you should be satisfied."

Groening agrees, with the proviso that there's "inside stuff" for die-hard fans. "And if you don't get it, you don't know the references, it's okay. But if you do know them, there's something extra there for you," he adds.

And there's probably something to offend people. The Simpsons is not without its critics. Groening reckons there's someone who's willing to pretend to be offended by everything. "We annoy people and that's part of the appeal. It's to entertain people and to annoy a certain segment of the audience as well."

Fox, with media mogul Rupert Murdoch at the head, always struck some as a strange home for The Simpsons. Jean points out the channel took a chance when no one else was willing to spend so much on an animated series. "And Rupert Murdoch played himself on the show. We wrote him a line, 'I'm the evil billionaire tyrant Rupert Murdoch', and he performed it with relish," recalls Groening.

Others who've guested on the TV show include Meryl Streep, Dustin Hoffman, Steve Martin, Susan Sarandon, Kirk Douglas, Jack Lemmon and Mel Gibson. Not everyone says yes. They've asked every ex-president and they've all refused. They've also pursued Bruce Springsteen with no luck.

"One of the great bits of fun we have in the show and in the movie is there's no style of comedy we can't tackle. We can do very sophisticated and wordplay and smart dialogue, and we can also do the hammer in the eye joke," says Groening.

'After 400 episodes - as Homer would say, one for every day of the year - it's harder to come up with new ideas, but we have a staff of writers, some of whom are so young they grew up watching the show. Al has a really good memory, but these younger writers really have memorised every episode and will say 'we did that in season six'."

Watching the film, he can see things he might have improved on but what he likes about the movie is that it's imperfect. "It's about the art of the hand-drawn gesture. It's not a CGI movie with 1,000 perfect penguins dancing in unison. We have one penguin..."

"...and something very bad happens to him," chips in Jean.

"In a way," continues Groening, "the movie is obviously a thrill for us because we get to listen to an audience in a theatre laughing. That doesn't happen at home with our families.

"The other thing is it's also a way of honouring the people that work on the movie. The actors are all fantastic. And it's a way of honouring the animators. This really is a tribute to the art of hand-drawn animation, which is basically disappearing. All the animated movies these days are computer-generated and this is the old-fashioned, hand-drawn, erase it, you didn't do it right, kind of thing."

Fans needn't worry that The Simpsons in the cinema heralds the end of the TV show. There's another season beginning on US television in September and Jean even feels the film may actually help the franchise by bringing people who've seen the movie to the TV series.

The Simpsons (PG) opens in cinemas on Wednesday.