When the show started it was provocative and outrageous. The first President Bush said that he wanted to see families "more like the Waltons than the Simpsons". When the show came on air, comedy was much softer and less self-referential. Every writer wants the opportunity to work on a show as well known as The Simpsons, but at the time we didn't know how eternal it would be.
When the show started, the writers were guys in their early thirties who saw themselves as Bart and their fathers as Homer; now they are in their late forties and identify with Homer. That's why we have more stories with Homer now.
For a long time in the early days people were conscious of not making Homer too dumb, because we thought people wouldn't warm to him, but that has changed and now he's as dumb as a post. They put in jokes about him being dumb and got laughs, and it went from there.
So much of writing is just looking around at one's own life. When I wrote the episode "Milhouse of Sand and Fog", I had three kids and was wrestling with chickenpox, so that was the nugget of the story.
One of the aspects of the job is being able to fight burnout, and there is certainly a turnover. The whole process of making an episode takes 10 months, and, as you've got 22 episodes in a season, you've got this going on simultaneously. Most sitcoms and drama shows have nine to ten weeks off a year, but not on The Simpsons. It is down to mutual support; there will be days when a writer just can't do it, and the others will have to step in.
The show's interest lies in identifying with the characters. You see other people in them: your boss is like Mr Burns; your brother is like Bart. People don't see themselves in it, though.
It started out as just the Simpson family, but now they've got a town of 500 characters that they can reach in and pick from. The past few years have seen a real desire from the writers to keep the show fresh. It tries hard not to be topical, and yet the Simpsons can't exist without cellphones and computers. If you look at Lisa's room now it has a computer in it, where it didn't before.
It has been a gentle commentator on world events for 20 years. The most challenging part is coming up with new storylines, and not repeating anything. The show is a testament to the resilience of the family. We've seen all kinds of issues thrown at the characters, and they still get up, go to work and school, come home and have dinner. They are still a family.
There were jokes invented on the show that were so popular they've now become hackneyed. You feel like you've put your own mark on something that will outlive you. The Simpsons has been the trunk of a tree from which many branches have grown. Now it's normal, and shows such as Family Guy and South Park push the envelope. As told to Rachel Shields
20 Simpsons statistics
20 years since the first episode
$527m (£324m) How much the 2007 Simpsons: The Movie grossed
448 episodes aired
9 Number of Simpsons comic-book series published since 1993
£1.5m Amount Tracey Ullman tried to sue Fox for a share of the profits
500-plus companies licensed to use The Simpsons in advertising
400,000 copies of the No 1 single "Do the Bartman" sold in the UK
544 special guest appearances
3 Number reached by "The Simpsons Sing the Blues" in the US
£246,000 How much the main voice actors are paid per episode
25 Emmy awards
£1.2bn Revenue The Simpsons merchandise reportedly generated in first 14 months of sales
2 Number of times the opening sequence has been changed
7.39 million viewers for the most recent episode of the new series
45 Number of languages Simpsons: The Movie translated into.
1 position in Empire's list of greatest TV shows of all time
20 writers work on each show
2 Number of eighth birthdays Lisa has had
1 billion commemorative stamps issued for the 20th anniversary
20 guest appearances
Tony Blair, November 2003
The PM acts as tour guide to the Simpsons, directing them to Parliament and Stratford-upon-Avon.
The Beatles, April 1991, September 1993, October 1995
All the living members of the band appeared on the show, in three separate episodes. Paul and Linda McCartney help Lisa to become vegetarian.
Michael Jackson, September 1991
When Homer is sent to a mental institution his cellmate is a large, white man who thinks he is Michael Jackson, voiced by, er, Michael Jackson.
Ricky Gervais, March 2006
The only guest star both to have written and appeared in an episode, Gervais plays a David Brent-alike who moves into the Simpsons' home as part of Wife Swap.
The Who, November 2000
"A Tale of Two Springfields" sees the city divided by a wall. It takes a concert from the English rock band on top of the wall to bring the two sides back together.
Elizabeth Taylor, December 1992
The actress provided vocals for baby Maggie's much-anticipated first word, "Daddy", a performance that was deemed too sexy and had to be re-recorded countless times.
Nicolas Sarkozy and Carla Bruni, November 2009
When Homer visits Paris he bumps into caricatures of Sarkozy and his wife, a femme fatale who throws herself at Homer's colleague Carl.
Elton John, February 1999
Elton is kidnapped by the men of Springfield, who believe that he is there to perform for Apu, whose declarations of love for his wife are showing them up.
Tom Jones, November 1992
Marge's favourite singer is kidnapped by a besotted Mr Burns in an attempt to woo her. He serenades Marge and Homer while chained in Mr Burns' basement.
Mulder and Scully, January 1997
The X-Files stars visit Springfield to investigate Homer's sighting of an "alien", which turns out to be Mr Burns, glowing green from years working in the power plant.
Dustin Hoffman, April 1991
Hoffman appeared under the pseudonym Sam Etic; a play on the word Semitic, alluding to the fact that both Hoffman and his character, Lisa's substitute teacher, are Jewish.
Stephen Hawking, May 1999
When Springfield is taken over by Mensa, the physicist intervenes to stop the smartest citizens from ruining the town.
Sting, January 1992
Bruce Springsteen was originally approached instead of the Police frontman for the Emmy-winning episode, which pokes fun at celebrity charity singles.
Johnny Cash, January 1997
The legendary singer voiced the part of an imaginary coyote conjured up by Homer after he eats Guatemalan insanity pepper.
Alec Baldwin and Kim Basinger, November 1998
Pre-split, the glamorous Hollywood couple appeared as, erm, a glamorous Hollywood couple hiding out in Springfield.
Mick Jagger, November 2002
The Rolling Stones singer appeared in a line-up of rock royalty including Elvis Costello, Lenny Kravitz and Tom Petty – all staff of the Rock 'n' Roll Fantasy Camp that Homer attends.
Spinal Tap, April 1992
The mock-rock band come to Springfield for a concert and inspire Bart to take up the guitar, but are run off the road by the school bus and seemingly killed in a blaze.
Gore Vidal, November 2006
More than nine million viewers watched the first screening of the episode, featuring Vidal alongside Tom Wolfe, Michael Chabon and Jonathan Franzen.
Eric Idle, February 2007
The Monty Python star plays a documentary film-maker who makes Homer feel embarrassed about his lack of achievement when he makes a film about Springfield residents.
The cast of Cheers, December 1994
When Homer is banned from Moe's he finds a new bar, and stumbles into the cast of Cheers, except Kelsey Grammer, who already had a regular role in the show as Sideshow Bob.
20 celebrity fans
1 George Takai 'Live long & prosper'
2 Hugh Hefner 'He's even been on it'
3 Dr Rowan Williams 'Good sense'
4 Simon Cowell 'My favourite!'
5 Ricky Gervais 'Best ever comedy'
6 Jedward twins 'Bart's best friends'
7 John Updike 'Well-disposed'
8 U2 'Asked to be in show'
9 Michael Jackson 'A guest voice'
10 Tony Blair 'Appeared during war'
11 Ralf Little 'I love it'
12 Richard Madeley and Judy Finnigan 'Watvh every night'
13 The Ramones 'Gigantic fans'
14 A N Wilson 'Brilliantly funny'
15 Susan Boyle 'It's got talent'
16 Sara Cox 'I love The Simpsons'
17 Simon Pegg 'Razor-sharp treatise'
18 Mark Leckey 'Social comment'
19 J K Rowling 'Has so many layers'
20 Morgan Spurlock 'Old fan'
20 key characters
Montgomery Burns: The evil owner of Springfield power plant is the embodiment of corporate greed, and can never remember employee Homer's name.
Ned Flanders: The Simpsons' God-fearing neighbour manages to infuriate both Homer and Reverend Lovejoy with his dogmatic devotion to doctrine.
Apu: Originally an illegal immigrant, Apu has a PhD but enjoys running the Kwik-E-Mart, and his wife and family from an arranged marriage.
Moe: The owner of Homer's favourite bar, Moe is frequently subjected to Bart's prank calls.
Barney Gumble: Always belching and usually with a beer in hand, Homer's best friend is Springfield's resident drunk.
Kent Brockman: The city's news anchor and entertainment presenter, the ubiquitous Brockman turns up at every Springfield happening.
Waylon Smithers: Mr Burns' adoring assistant, devoted to his boss and his Malibu Stacey collection.
Chief Wiggum: The obese, dim-witted chief of Springfield police is corrupt – often taking bribes – but usually well-intentioned.
Groundkeeper Willie: The flame-haired Scottish groundkeeper at Springfield Elementary School who is always losing his temper.
Nelson Muntz: The school bully, Nelson, and his gang terrorise most of the other children, usually to the tune of his "Haw-haw" laugh.
Milhouse Van Houten: A bespectacled geek constantly being bullied at school or led into trouble by his best friend, Bart.
Ralph Wiggum: Bart's oddball classmate is often seen chewing on grass, paint, and other inedible substances, and is the target of bullies.
Krusty the Clown: Bart's favourite TV clown, rarely seen without his make-up, who is really a miserable, alcoholic smoker.
Principal Skinner: Vietnam veteran and principal of Springfield Elementary School, Skinner is constantly battling with Bart.
Reverend Lovejoy: The tolerant minister at the First Church of Springfield admits that all religions are much the same, and that evolution probably happened.
Sideshow Bob: Krusty's sidekick, the curly-haired criminal mastermind is voiced by Frasier star Kelsey Grammer.
Grampa Abraham Simpson: Homer's rambling father, who has a fractious relationship with his son.
Itchy & Scratchy: The TV cat and mouse pair whose over-the-top violence was so popular that they got their own Simpsons episodes.
Selma and Patty: Marge's raspy-voiced, chain-smoking twin sisters are united in their hatred of Homer.
Mayor Quimby: Springfield's slimy mayor takes kickbacks, seduces women and doesn't care about his city, which he called "Springfeld" in one speech.
Bart's 20 best blackboard lines
I will not waste chalk
I will not sell school property
Mud is not one of the four food groups
All work and no play makes Bart a dull boy
A fire drill does not demand a fire
The Pledge of Allegiance does not end with Hail Satan
"butt.butt" is not my email address
I cannot absolve sins
I will not yell "She's dead" during roll call
I will return the seeing-eye dog
I will not charge admission to the bathroom
Organ transplants are best left to the professionals
Indian burns are not our cultural heritage
Pork is not a verb
I won't not use no double negatives
I did not win the Nobel Fart Prize
I cannot hire a substitute student
This school does not need a "regime change"
SpongeBob is not a contraceptive
Global warming did not eat my homework
"Beer: The cause of, and solution to, all of life's problems."
"Kids, you tried your best and you failed miserably. The lesson is, never try."
"What's the point of going out? We're just going to wind up back here anyway."
"If you don't like your job you don't strike. You just go in every day and do it really half-assed. That's the American way."
On the USA:
"America's healthcare system is second only to Japan ... Canada, Sweden, Great Britain ... well, all of Europe. But you can thank your lucky stars we don't live in Paraguay!"
"Son, a woman is like a beer. They smell good, they look good, you'd step over your own mother just to get one! But you can't stop at one. You wanna drink another woman!"
"Don't let Krusty's death get you down, boy. People die all the time. Just like that. Why, you could wake up dead tomorrow. Well, good night."
"How is education supposed to make me feel smarter? Besides, every time I learn something new, it pushes some old stuff out of my brain. Remember when I took that home winemaking course, and I forgot how to drive?"
"I'm going to the back seat of my car, with the woman I love, and I won't be back for ten minutes!"
"You're everywhere. You're omnivorous."
"Facts are meaningless. You could use facts to prove anything that's even remotely true!"
On life skills:
"Weaselling out of things is important to learn. It's what separates us from the animals ... except the weasel."
"Dad, you've done a lot of great things, but you're a very old man, and old people are useless."
"If you really want something in life, you have to work for it. Now quiet – they're about to announce the lottery numbers."
"Stealing? How could you?! Haven't you learned anything from that guy who gives those sermons at church? Captain what's-his-name? We live in a society of laws. Why do you think I took you to all those Police Academy movies?"
"Son, when you participate in sporting events, it's not whether you win or lose: it's how drunk you get."
On mental health:
"What do we need a psychiatrist for? We know our kid is nuts."
"What if we've picked the wrong religion? Every week we're just making God madder and madder?"
"Fame was like a drug. But what was even more like a drug were the drugs."
On the supernatural:
"Lisa, vampires are make-believe, like elves, gremlins and Eskimos."
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