IT'S hard to imagine a world without The Simpsons.

Not only is it arguably the greatest cartoon series of all time, it's also one of the best sit-coms. The antics of Homer, Marge, Bart, Lisa, and Maggie, along with an array of fellow oddballs in Springfield, have been keeping us entertained now for nearly 20 years.

The Simpsons has become so ingrained in our cultural landscape that "D'oh!", Homer's catchphrase, is listed in the Oxford English Dictionary. Even a United States president has made reference to the show, albeit critically. "The nation needs to be closer to the Waltons than the Simpsons," said George Bush senior, back in 1992. The show's creator Matt Groening replied: "Hey, the Simpsons are just like the Waltons. Both families are praying for the end of the Depression."

Since The Simpsons first hit our screens back in 1989, there have been 449 episodes produced and last year the world's favourite dysfunctional family finally made it to the cinema in a feature-length film.

Over the years the show has won two dozen Emmy awards and attracted such legendary and diverse guests as Bob Hope, Sir Paul McCartney, Stephen Hawking, Tom Hanks, Lance Armstrong, Tony Bennett and JK Rowling, to name but a few.

It has also paved the way for other satirical animation shows such as South Park, Family Guy and King of the Hill, although none has come close to stealing its TV crown. As the show's popularity has grown, characters like Mr Burns, Ned Flanders and Crusty the Clown have become household names. But what about the team of writers working feverishly behind the scenes?

Al Jean has been a writer and producer on The Simpsons since it started, and was among the guest speakers at the 15th Bradford Animation Festival, hosted this month by the National Media Museum.

He admits that the show's phenomenal success took everyone by surprise. "We thought it was good, but it's almost defying gravity to be on TV 20 years," he says. "Having said that, I'm as proud of the 20 episodes we've recorded this year as I am of any series we've ever done. The pace is a little faster, but if you take an episode from today and put it next to one from 15 years ago you wouldn't notice the difference."

So what's the secret of its unparalleled success? "Everyone comes from a family like the Simpsons and it's so appealing because even a three-year-old can understand what's going on. The characters resonate all around the world, it doesn't matter what language it is you can put Spanish words in their mouths and people love it," he says. "As a kid I would watch Rocky and Bullwinkle and think it was great. Then I'd watch it as an adult and get different jokes and that's what we always try and do with The Simpsons."

Jean's comedy writing career began 30 years ago while he was still at college. "When I was in high school I read a lot of the great works of literature, including Dostoevsky and Dickens, and then I got on to the Harvard Lampoon magazine, which was very cool, and it
went from there," he says. "I would watch shows like Saturday Night Live and I realised that someone like me could get a job on TV."

But if he thought TV comedy writing was a piece of cake, he learnt a valuable lesson very early on. "I've been fired once in my life, and that was because I was too quiet, so I made sure that would never be the reason I got fired again," he says. Jean went on to work as a writer on The Tonight Show and then as a producer on It's Garry Shandling's Show, before being poached by Matt Groening and James L Brooks.

When he started on The Simpsons it was little more than a short gag-fest, but it quickly evolved into a show with detailed storylines and an array of characters brought together by a team of 20 writers. "A script will be re-written seven or eight times before the cast read it, because if just one person laughs at a joke then we're out of business," he explains. Which means it can be a lengthy process. "I rem-ember we did a joke once about the Soviet Union and by the time it was actually shown the Soviet Union had broken up."

During his time on the show Jean has been closely involved in more than 200 episodes, including Moaning Lisa, Stark Raving Dad and a hilarious Mary Poppins spoof. He's also met some of the most famous people on the planet. "It's a crazy list of people who've appeared on the show. Elton John was someone I admired since I was a kid and he completely lived up to what I'd hoped. He even did a song for the show."

"Tony Blair was another once-in-a-lifetime experience, I couldn't believe I was sitting in 10 Downing Street talking to the Prime Minister." The Simpsons even attracts renowned literary figures. "We managed to get Thomas Pynchon, who never does anything, to come on. He has a son who was a fan and that's why he did the show He was great – you wouldn't have imagined he was supposed to be this reclusive figure."

Jean admits that attracting such stellar names has become easier over the years. "Once the show became a success it became a cool thing to do – although we found it easier if they had children because then we could bribe them with Simpsons merchandise.

"As well as being a scriptwriter and producer, Jean has helped come up with some of the show's characters. "Ralph Wiggum was one I helped create. In his case, we wanted a boy who was like Homer and afterwards he just took on a life of his own. But that's often how it works; we don't say we're going to introduce a character, we just do an episode and then if a character catches on we'll bring them back."

Just like any great satire, The Simpsons occasionally sails close to the boundaries of good taste, but Jean says so far they've managed to avoid offending the censors.

"Sometimes we get a message from the TV network saying Homer shouldn't do this or that because it sets a bad example, but our answer is that 'Homer is a bad example'. We take stereotypes to an insane degree and the whole point is that we are being ridiculous."

So is there anything that's deemed out of bounds, even for The Simpsons? "There are certain things that are so tragic like 9/11 where there wasn't a way, for me, of making it comedic. But nonetheless we've done jokes about Bin Laden and there is the old idea that tragedy plus time equals comedy. Certainly the assassination of Abraham Lincoln was tragic, but now it can be used in comedy," he says.

"Also we've shown Homer naked more times than you could if he was a real person and we've shown things which if you filmed as live action would be horrific, but the fact it's animation is a saving grace."

Fans of the show will be glad to hear that the cast have signed up for at least another three series and there's no sign of it coming to an end. "I'm always amazed wherever I go in the world at how many people know about The Simpsons and love the show," says Jean.

"It's very easy to overstate the influence of entertainment or pop culture, but I think (the show) is one of those iconic things that's as big as anything."

Long may it continue.