The Inbetweeners: What's next for Britain's oldest schoolboys?

They are nearly an hour late, and I'm about to abandon all hope when, finally, they appear, slouching and rubbing their heads like the nation's favourite hapless teenagers. I half expect the foursome to be swapping crude insults and bottom jokes, like the schoolboy caricatures they portray in Channel 4's hit series The Inbetweeners. Instead, they mumble apologies and file sheepishly into an empty room in a house used for location shots in north London.

They are all well into their twenties, school a distant memory, though in the public imagination they remain fixed for ever as Will (Simon Bird, 26), Jay (James Buckley, 23), Simon (Joe Thomas, 27) and Neil (Blake Harrison, 25). What started as a cult show with barely 200,000 viewers two years ago has turned into a television juggernaut with more than three million watching the third – and almost certainly final – series, which ended this month.

So, what do they do for an encore? How do they match the zeitgeist of the show? Well, a feature film is in production: The Inbetweeners on a lads' holiday to Malia, Crete. And this weekend it emerged, to the relief of their fans, that there will be a two-part TV special for Christmas 2011.

They will have to go some to match some of the highs – lows? – of the series: Will soiling himself during exams; a field trip ending with Simon naked in front of shocked classmates; and Neil punching a fish to death. Scenes described by one critic as "sexist, stupid, deluded, ill-mannered, unbelievably vulgar and terribly funny".

One of the most compelling elements in the show is the boys' obsession with sex. Today, it appears to have bled into their off-screen lives. "We were talking about when we first lost our virginity the other day," Bird blurts out. "We know that Joe's was on Christmas Day but he wouldn't tell us which year." Joe retorts: "I think I was 18, actually, when I lost my virginity so I think I'm in the majority." Bird confesses his was "later than normal but earlier than weirdly late". Harrison simply mutters: "Average... average as in nothing to say." Buckley reduces them all to hysterics with: "I can't wait... and the first thing I want to do is to tell a newspaper about it."

Despite the rapport between the actors and the popularity of the show, repeating a winning formula was difficult, says Bird. "The show had grown so much between the first series and the second series. It's weird making a show knowing it's going to be a hit; it puts more pressure on you."

Buckley admits to having had doubts: "In the back of your head you do think, 'we're just making each other laugh and mucking about ... will anyone else find this funny?'"

Evidently, they do. DVD sales have topped a million – 23,000 last Monday alone, when copies of the latest series hit the shops – confirming the show's transition from cult favourite to mainstream success.

And it has added to the wealth of the English language – well, its store of swearwords anyway, "bumder" and "briefcase wanker" being two of the more printable.

Slightly disappointed by the fact that none of them has uttered a single profanity, I ask how many of those featured in the show they came up with. Well, er, none, Thomas explains, a little sheepishly: "People often think it's improvised, but actually it's quite strictly scripted. We're not really allowed to go off script very much." To the others' amusement he adds: "I mean, partly that's because we don't learn our lines in the first place... we have no moral high ground."

While Bird doesn't rule out another series, he is worried about being typecast. "I'm taking control of trying to not be overexposed... I just want to make comedy, really, and whatever I can do to extend my career doing that."

This isn't something that concerns Buckley. "I've never worried about it. I'm just an actor, and although the last two years what I've done has been comedy, I'm not particularly funny. I just do whatever I like and whatever's written in front of me, and I'll carry on doing that."

Buckley and Harrison, products of stage school, regard themselves as actors first and foremost, whereas Bird and Thomas are Cambridge University graduates and veterans of the Footlights, a proving ground for comics from John Cleese and Stephen Fry to Sacha Baron Cohen.

The pair will star in Chickens, a new comedy series they have co-written for Channel 4 that sees them play conscientious objectors during the First World War. Harrison is set to appear in a new comedy, The Increasingly Poor Decisions of Todd Margaret, while Buckley has signed up in the Del Boy role for two more specials of Rock and Chips – the prequel to Only Fools and Horses.

And yet, for all their talk of other projects and the "work" of The Inbetweeners, they inhabit – and are inhabited by – their somewhat pitiful on-screen counterparts. Bird relates how he broke his teeth on his desk: "I was making fun of somebody for doing a bad joke by fake laughing and then I smashed my teeth out and suddenly everyone was laughing at me."

While Bird and Thomas admit to being similar to their on-screen characters, Buckley claims to be more like Neil, with an "unaware naivety". Harrison channels the lovestruck Simon: "I had a massive crush on someone: it was very much an unrequited love." He apologises for not having any funny stories from his teenage years, saying he was too "safe and sensible".

The mood darkens as Buckley recalls someone spitting in his English book. "That was true, that happened. It wasn't particularly funny, it was just something that happened to me. It made me cry... I was about 12."

Thomas breaks the awkward silence: "That's bad, actually." Then abruptly changes the subject: "There was a girl that I fancied who was kissing me, and I was still too nervous and paralysed with fear and doubt. I thought that if I tried to kiss a girl she would call the police." The group dissolves into laughter.

The Inbetweeners file out, shaking my hand and mumbling: "bye", "nice to meet you". For a brief moment they are schoolboys once more. And they still haven't sworn once.