If there's an image that sums up Dumb and Dumber - puerile, vulgar, undeniably hilarious - it's Jeff Daniels on the toilet, expelling the effects of a heady laxative dose. It's less the gag itself - that Jim Carrey's character has spiked him in an act of petty revenge - than Daniels' commitment to the idiocy: eyes almost crossed with relief, scarecrow-like hair in all directions, tasselled buckskin boots in the air.
Bennett Yellin, who co-wrote the film with the Farrelly brothers, Peter and Bobby, remembers that when they screened an extended rough cut of the film, in which the toilet sequence was even longer, there was one person who didn't find it funny at all: Jeff Daniels' manager. "His manager turned to me, ashen," recalls Yellin, laughing. "He said, 'You're cutting this out, aren't you?' He was horrified."
Daniels' management had already tried to stage a crisis intervention to stop the usually-serious actor taking the role of Harry Dunne, best friend to Carrey's Lloyd Christmas. "Twelve-year-old boys are going to see it, and that's it," Daniels' reps warned him. But the actor's instincts had been absolutely right about Dumb and Dumber. Carrey was on a box office hot streak that would make him (for 1996's The Cable Guy) the highest paid actor in Hollywood history, while Dumb and Dumber itself would be a zeitgeist-shifting hit.
One of the most important comedy films of the Nineties, Dumb and Dumber made political incorrectness and gross-out gags the in-thing. Thirty years since it was first released, it's still stupidly funny.
"I remind myself there have been a lot of movies that have won Academy Awards over the years but nobody ever talks about them," says Yellin. "They don't quote them. People have come to embrace this one for some reason."
Interestingly, Dumb and Dumber was almost a John Hughes film. At the time, Yellin and the Farrellys were a jobbing screenwriting trio, while Hughes had a production deal at Universal. Hughes saw one of their scripts - an unproduced sequel to the Tom Hanks-Dan Aykroyd caper Dragnet - and asked them to pitch him an idea.
"We came back and pitched him Dumb and Dumber," says Yellin. "He said, 'I love it! Write it and if I like it, I'll let you direct it.' We were like, 'What?!'"
The eventual film follows Lloyd and Harry as they drive across the country - from Providence, Rhode Island to Aspen, Colorado - to return a lost briefcase to out-of-their-league socialite, Mary Swanson (Lauren Holly). What they don't know is that Mary's husband has been kidnapped and the briefcase is full of ransom money. Lloyd and Harry make themselves unwitting (and witless) targets in the kidnapping plot.
But Hughes's production deal came to an end before the idea came to anything. "What happened - which is typical of production deals that filmmakers have - is that when it ended, everything went into a cold crevasse," says Yellin.
A few years later, they approached Hughes about taking the script back and trying to get it made elsewhere. "He said, 'Fine but you can't use my name to set it up. I'm not the producer of it. Or you have to pay me a million dollars,'" recalls Yellin. "We said 'OK!' and saved a million dollars on the project!" Though Yellin still considers Hughes "the unwritten godfather" of Dumb and Dumber.
At one point Steve Martin and Martin Short were attached to potentially star. The idea of Dumb and Dumber as a John Hughes movie with Steve Martin and Martin Short feels like something from a decade earlier. Indeed, what's significant about Dumb and Dumber is its role as a transitional Nineties comedy. It's a film that led the way - riding on a minibike with snot icicles plastered across its face - for the decade's less-than-PC, outrageous, shamelessly immature comedies. The Farrellys upped the ante themselves with the much-underappreciated Kingpin and hair-raising hit There's Something About Mary, which came alongside Adam Sandler's brand of shouty, man-child humour and the likes of American Pie.
The first big taboo-breaking laugh of Dumb and Dumber comes when Lloyd, needing to raise cash for their road trip, sells a decapitated budgie to a blind boy. Cue a scene of the boy in a wheelchair, stroking the budgie - its head sellotaped back on - and cooing, "Pretty bird, pretty bird."
Other big laughs are similarly boundary-pushing, or bodily function-obsessed. See Lloyd and Harry accidentally kill a man when they feed him red-hot chillis and rat poison; a traffic cop twitching after he swigs back a bottle of Lloyd's urine; Harry peeing down Lloyd's back when they're frozen to each other on the minibike; Harry's stretchy tongue stuck to a frosty ski lift - one of the great gross-out comedy images - or Lloyd's near-sexual assault when he visits the wrong public toilet at the wrong time ("For manly love be here March 25th at 2:15am sharp" says the cubicle wall graffiti, at which point Lloyd looks at his watch and sees that it's March 25 2:14am - a superb gag).
Yellin says they weren't thinking about what comedy was doing at the time, or trying to be deliberately un-PC. They just wrote what made them laugh.
"That's our sense of humour! That was just us!" he says. "Those were the kind of jokes we told each other. There was never a conscious intention to be this or that."
The only thing that was deliberate was modelling their story on Bing Crosby and Bob Hope's series of Road To movies. "From the beginning I said this is our Road To movie," says Yellin. "The relationship between the two guys is that they're friends but they will stab each other in the back for the girl. That was as deliberate as we got. We watched several Bob Hope-Bing Crosby movies - that was the template."
As funny as Dumb and Dumber ultimately was, studios weren't interested at first. The script was rejected repeatedly - to the point they renamed it and resubmitted it, something they did more than once. "It went three times around," says Yellin. "Once as Dumb and Dumber, once as Go West, and once as A Power Tool Is Not a Toy [named after a song by The Young Adults]."
One hindrance, admits Yellin, was the fact they'd sent the script with Peter Farrelly's name attached as director. "Which was a ridiculous thing," says Yellin. "He had no credits whatsoever. He was just another writer with a few things produced. Attachments are supposed to help the film get set up, not help it not get set up."
It was Jim Carrey's name that got Dumb and Dumber made. Carrey saw the script while filming rubber-faced blockbuster The Mask at New Line Cinema. "Once Jim Carrey got attached, suddenly New Line wanted to be involved," says Yellin.
Indeed, Carrey was the comedian of the moment. Between Ace Ventura: Pet Detective and The Mask, also released in 1994, Carrey had already notched up $450 million-plus at the box office that year - all before Dumb and Dumber hit cinemas.
In preparation for meeting Carrey, Yellin went to see Ace Ventura at the cinema. Taking in Ace Ventura's pointed hair, over-exaggerated catchphrases ("Al-righty then"), and manic limbs, Yellin remembers thinking, what the hell?
"I was trying to imagine him as Lloyd," says Yellin. "In Ace Ventura, that character could not be more extreme - the voice, everything. But the first thing Jim said when he shook my hand was, 'I just want to assure you I'm not doing Ace Ventura.' He didn't know that's what I was thinking but it totally put me at ease!"
Jeff Daniels later described how Carrey didn't want another comedian to co-star as Harry, because two comedians would be constantly trying to top each other. Instead, Carrey wanted an actor to focus the buddy-buddy dynamic. At different points, Carrey wanted Nicolas Cage - who opted to make Leaving Las Vegas instead - or Gary Oldman. "In retrospect that would have been great!" says Yellin.
The Farrellys and Yellin had liked Jeff Daniels in Jonathan Demme's crime comedy Something Wild, but Daniels was still best known for dramatic roles in Terms of Endearment and Gettysburg. And Daniels' management weren't the only ones who didn't want him to play Harry. New Line Cinema executives weren't sold on Daniels and offered a low salary to put him off the project.
"They didn't want Jeff Daniels because they couldn't see what we saw," says Yellin. "They absolutely didn't want him to do it. New Line lowballed him. But he wanted to change things up and he wanted to work with Jim Carrey."
Speaking in 2014, Peter and Bobby Farrelly recalled that while Carrey used his box office hot streak to negotiate a salary of $7 million, Daniels was so determined to make the film that he accepted New Line's offer of just $50,000.
Daniels later admitted that he struggled to get the character right in rehearsals but it clicked when he made some internal decisions about Harry: that he had an IQ of eight ("not seven, not nine - eight") and that his reactions were always delayed by half a second.
The dynamic between Carrey and Daniels hinges on one of those rare, instant chemistries - one that manifests as an escalating stupidity in brightly-coloured tuxedos. There is a warmth between the characters - and not just from Harry peeing down Lloyd's back - as Carrey and Daniels' real-life friendship comes through in Lloyd and Harry: antagonistic, co-dependent, pea-brained soul mates. Carrey also brings a sweet pathos as Lloyd (aside from the snide laxative dosing, that is), pre-empting more dramatic turns in The Truman Show, Man on the Moon, and Eternal Sunshine of the Spotless Mind. "He's got chops!" says Yellin.
And while Dumb and Dumber is well remembered for its toilet humour and slapstick set-pieces, the script counters those moments with precision perfect jokes. Especially hilarious is Lloyd practicing his big romantic speech to Mary - "I feel like a schoolboy again, a schoolboy who desperately wants to make sweet, sweet love to you" - then blurting it out all wrong when the moment comes. "I desperately want to make love to a schoolboy." Also, Harry's response when Mary says she doesn't want to bore him with her personal problems. "Thanks," Harry replies.
Yellin had an inkling the film would do well. When The Mask was released in July 1994, he would visit the cinema just to see the reactions to the preceding Dumb and Dumber trailer. "You can never know what's going to be a hit," says Yellin. "If you did, you'd make hits all the time. But I went in to watch people react to Dumb and Dumber and I went, 'Oh my god, we're in the right place, at the right time, with the right movie, because they are roaring. I think we've got a hit here...'"
Released on December 16, 1994, Dumb and Dumber made $246 million worldwide and found an even bigger audience on home video - though critics in the US weren't laughing. Yellin's mother kept one review to put on the fridge: "I don't know where it came from," says Yellin, "but she highlighted a line that said, 'The writer's mother should be ashamed.'"
Dumb and Dumber is perhaps so of its moment - that moment when Jim Carrey became the name in Nineties comedy - that the belated sequel, Dumb and Dumber To (2014), didn't connect in the same way. "It's hard to be funny in a way that something was funny 20 years earlier," says Yellin.
The original, though, still has its tongue stuck to the cultural consciousness with its tuxedos, dog-shaped van, and much-repeated lines ("So you're telling me there's a chance...") It's largely bedded in from childhoods spent watching Dumb and Dumber on VHS. "It constantly blows my mind," says Yellin about the film's popularity 30 years on. "We just wrote a movie that would make us laugh."