Meeting Peter Farrelly is somewhat disarming.

With his goatee and shoulder-length hair, he looks more like a hippie maths teacher than a Hollywood funnyman. But then, together with his brother Bobby, Farrelly has been responsible for some of the most side-splitting movie moments in the past two decades.

Remember the hair-gel scene in There's Something About Mary? Or Woody Harrelson paying his rent "in kind" with his landlady in Kingpin? Not since the days of Animal House had American cinema been so raucous, reviled and refreshed at the same time.

In the 1990s, at least, the Farrellys were the undisputed kings of comedy - putting a new spin on the phrase box-office gross. Their 1994 debut Dumb And Dumber, in which Jim Carrey and Jeff Daniels played two idiots on a road trip to Aspen, set the tone: made for $17 million, it took $247 million around the globe. If critics carped that their humour was based around shock value, the studios didn't care: after all, the Farrellys' particular brand of shock was rather good value, you might say.

Still, compared to their early years, it's been a rocky period for the brothers in an era when Judd Apatow and his acolytes have stolen their thunder. Films like Hall Pass and the risible reboot of The Three Stooges have seen audiences dwindle - though it hasn't changed their approach.

"We like doing comedies," shrugs the 57 year-old, today on show without his younger sibling in tow. "They haven't always done as well as each other. But it's what we want to do. It's like a painter, doing a painting. You've just got to go with what's coming out of you at that time."

And that means Farrelly and his brother are back this month with Dumb And Dumber To, a long-gestating sequel to their debut which reunites Carrey and Daniels as the brain-dead Lloyd Christmas and Harry Dunne 20 years on from the original. Another road-trip - this time to El Paso - flush with toilet humour, the critics in America rather predictably savaged it. "I wish I could put as little thought into writing about Dumb And Dumber To as the Farrelly brothers did in making it," said Time magazine, in one of the kinder comments.

Yet the Farrellys are having the last laugh, with what's proving their biggest US hit since 2001's Shallow Hal, grossing $78 million so far (with a worldwide total of $116 million to date, it should outstrip 2007's remake The Heartbreak Kid, the last time the brothers broke the $100 million barrier). It also scored Jim Carrey's biggest live-action opening weekend since 2003's Bruce Almighty, doubtless injecting some much needed mojo into a career that has been in gradual decline since he made the brilliant Eternal Sunshine Of The Spotless Mind a decade ago.

While the brothers were not involved with the execrable 2003 prequel, Dumb And Dumberer: When Harry Met Lloyd (which starred Eric Christian Olsen and Derek Richardson as the young Lloyd and Harry), they'd always dreamed of returning to their first ever hit.

"It's the only movie we ever wanted to do a sequel to," says Farrelly. "We could've done There's Something Else About Mary. And we could've done a sequel to other things. But they didn't feel right, it didn't feel natural. Those movies were over. But in Dumb And Dumber, you left those guys exactly where you started with them earlier in the movie, and it seemed like you could do that a few times."

Back when they made the original, the Farrellys had zero feature experience; just two episodes of Seinfeld to their names and a 1987 TV movie showcase for comic Paul Reiser that Peter co-wrote with Bennett Yellin, who later went on to help pen Dumb And Dumber. He can still remember how difficult it was selling their debut.

"If I pitched you the first one before you'd seen it, and I said 'Two dumb guys drive over to Aspen because one of them is in love with a girl', you'd say 'Pete, don't f****** make it. It sounds horrible.' But that's what Dumb And Dumber is; it's not about the plot, it's about the characters."

This time around, the story sees Lloyd and Harry hunting down the latter's daughter that he never knew he had - with the express intention of getting her to donate a kidney. It's not exactly Chekhov; more an excuse to keep the gross-out gags coming at the speed of sound. Fans will be happy with nods to the original - from Petey the blind parakeet owner to the dog-grooming 'Mutts Cutts' van - though its debatable whether the Farrellys' risqué humour is as transgressive now as it seemed two decades ago.

Still, one thing is certain: seeing Carrey back to his old gurning self is pleasing. It's the Farrellys' third time working with him, after he played a cop for them in Me, Myself And Irene. "Jim is the best," enthuses Farrelly. "He's a Bill Murray. He can do anything. He's also a great writer. Most actors don't really bring that much to the script, a couple do, but Jim takes that script and he goes from page one to the end, and he adds lots and lots of funny, funny things. He's a guy you welcome ripping the script up."

Aside from having Carrey batting for your team, it helps when you get on with your co-director. With just an 18-month age gap between them, and both long-term married with children, it's always been like this since they were born and raised in Cumberland, Rhode Island, the sons of Robert (a doctor) and Mariann (a nurse). "We grew up in the same bedroom, and we used to fight all the time, but as soon as the fight's over, boom, it's all good. We never had grudges. And that's how it's been. When you have your little blow-ups, it ends quickly."

While they've often been seen as the natural successors to Jim Abrahams and David and Jerry Zucker - the team behind Airplane!, a seminal film for the Farrellys - that didn't arrive until their mid-twenties. "We didn't have a movie theatre in our town when we were growing up," says Farrelly. "We were about 25 minutes from the city in Providence, Rhode Island. And in our town there was no movie theatre, so we only went to one or two movies a year. If The Sound Of Music came out, we'd go! So most of our influences were the guys around us; there were lots of funny guys in our neighbourhood for some reason."

Then there was the television. Farrelly cites The Andy Griffith Show, which starred Griffith and a young Ron Howard in a story of a small town Sheriff. "You always were sure to laugh, but also there was always a little heart in there. And you really cared about the characters. When we got into the business, and were finally able to make our first movies... we thought, 'What if you can have a broad comedy where you're laughing your ass off for 90 minutes but you also really care about these people? You really want to see what happens to them.'"

This brand of sweet and sick - heartwarming one minute, stomach-churning the next - has guided the Farrellys ever since they moved to LA in the mid-1980s, with the dream of pursuing writing careers. In the case of Peter, he's also produced two novels, Outside Providence and The Comedy Writer, a semi-autobiographical tale of a New England man who quits his sales job to become a screenwriter in LA. While it was comedic, it was also serious. "I do have that side in me," he says, as if to highlight that he's not just a comedy writer.

Will they ever make a drama? "Eventually we will probably make something more serious," he nods. "There's no grand plan to our careers. It's not like we can see three or four movies in a row, where we're going to be going. Basically, what happens is you finish a movie and you look at where you are, and what you want to do, and you just go with the thing that's sort of there. We've never held back a serious film because we wanted to do a comedy. It just hasn't been there but I guarantee it will come."

Given the success of Dumb And Dumber To, that may be a while yet.

Dumb And Dumber To opens on Friday