The donkey sex scene midway through the movie does the trick. Up until then The Heartbreak Kid might have been just another screwball comedy, complete with an amiable central turn from Ben Stiller as the hapless honeymooner Eddie and zany support from Malin Akerman as his new, and increasingly terrifying, wife Lila. However, a setpiece involving a woman, a donkey and an enormous animal erection announces unequivocally that “gross-out” comedy is back, and, more importantly, that its chief purveyors – Bobby and Peter Farrelly, the co-directors of Heartbreak Kid – are back too. And this time they’re grosser than ever.

“That was actually a female donkey with a strap-on penis,” says Bobby Farrelly, sitting in a London hotel room and speaking calmly and candidly, without a flicker of smut in his voice. “It took two weeks to train her, but the very first take that she gave us made us laugh our heads off. So it was worth it in the end.”

The Farrelly brothers, of course, need a hit. The former salesmen and self-taught film-makers made their names and invented the gross-out genre itself with the risqué box-office smash There’s Something About Mary (1998). A simple story about a thirtysomething nerd (Stiller again) in search of his former highschool sweetheart, it introduced mainstream cinema to sperm gags (remember the “hair gel” scene?) and created an international market for movies such as American Pie, Road Trip and all their semen-spattered epigones.

And yet, while the gross-out movie evolved and intensified, the Farrelly movies seemed to get lighter and softer, until in 2005 they made their softest movie yet, the PG-rated romance Fever Pitch. “Everyone expected us to go further and further with the comedy,” says Farrelly. “But we didn’t want to do what was expected.”

Unfortunately, their dwindling penchant for perversity seemed to be in direct proportion to their box-office return – Fever Pitch pulled in a paltry $42 million (£20.5 million) in the US, compared with Mary’s $176 million.

Furthermore, while the Farrellys were struggling with sentiment, a new class of ribald comedy kings had risen. Judd Apatow and his ensemble made smash hits such as Knocked Up and Superbad (both 2007), foul-mouthed, sex-obsessed movies that seemed to snatch the gross-out crown from the Farrellys.

The Heartbreak Kidis thus the Farrelly brothers boldly returning to home turf. The brothers are here, flexing their well-honed comedy muscles, showing the world, and especially Apatow, that the real kings are back. No? “Well, not intentionally,” says Farrelly. “I like Judd Apatow. I don’t know him and I don’t know any of the actors in his movies. But I think that he’s doing very original, fresh stuff, and I’m all for it. I don’t think he’s taking any of the market away from us.”

But what about the timely return, after the soft sentimental lull, to gross-out itself? “Look, we enjoyed making the sentimental movies just as much as anything,” he says. “But we felt, comedically, that we were a little bit handcuffed. So when this one came up we knew that there would be sex and adult situations and areas that we hadn’t joked about in a while.”

However, Farrelly adds that gross-out comedy isn’t just a directorial option for him or a creative choice. It’s something that’s in his blood, and goes right back to his childhood in Rhode Island with his brother and three sisters. “There was a kid in the neighbourhood who said that he could light his farts on fire. And we were like: ‘You can not!’

“And so he put a match to his ass and farted, and ssshhhfffeeewwwww, a big flame came out.”

Farrelly giggles involuntarily to himself before adding: “I had never seen anything that made me laugh so hard, and I don’t remember seeing many things in my life that I found funnier than that since.”

After college he and Peter drifted into salesman jobs, the former selling life insurance, the latter shipping space. Dissatisfied and bored with their lot, they bought a screenwriting manual and began writing comedies in their spare time. When their first script, Dust to Dust – “about a couple of goofy guys in a funeral parlour” – was bought by Eddie Murphy’s production company in the early 1980s, they began a fruitful decade as professional screenwriters.

“We made a little cash and we were having a good time, but after ten years it became frustrating that ultimately nobody was actually making these comedies that we were writing. So we decided to direct them ourselves.”

Their debut movie, Dumb and Dumber (1994), was an instant $130 million smash, but it wasn’t until Mary, four years later, that the Farrellys truly became part of the Zeitgeist. “I remember at one screening turning around and seeing a 75-year-old woman laughing just as hard as the 16-year-old boy beside her,” he says. “Right then I knew that we’d struck a nerve.”

The Farrellys, unsurprisingly, had their detractors too. They were regularly accused of insensitivity to minorities – they laughed at schizophrenia in Me Myself and Irene (2000) and at obesity in Shallow Hal (2001). More importantly they were often said to be responsible for the dumbing down of popular culture (indeed the phrase “dumbing down” was coined after Dumb and Dumber). Farrelly, who’s currently working on a long-gestating Three Stooges movie, groans at the mere mention of the dumbing down debate. “I dunno. If our movies seem dumb then it’s just because the world seems dumb,” he says. “Comedy goes to where society brings it.”

He adds that the gross-out comedies work not because of how dumb they are, and not because of a preponderance of lowest-common-denominator sex gags. No, he says, they are successful because, in their essence, beneath all the bodily fluids, they are sweetly familiar romantic fables. “After every movie people come out talking about the big outrageous profane gags,” he says. “But if you took away all those moments it would be like: ‘Oh, it’s the story of a guy who loves a girl, loses her and gets her back. Hey, I’ve seen this a thousand times!’ ” Farrelly allows himself a wry chuckle and adds: “So, it’s just a way of disguising a very sweet story. Nothing more.”

The Heartbreak Kid is released on Thursday 4 October, 2007

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