"I don't know that we'll necessarily top the first movie," says Thomas Ian Nicholas, a star of the American Pie teen-film series, which started in 1999 and will next month expand to a fourth instalment, American Pie: Reunion. "But I feel this is the best sequel."

It must be tricky territory for the franchise's five leads; bittersweet. On the one hand, the thirtysomethings clearly delight in one another's company, tirelessly joking and jostling in a London hotel. On the other hand, none would have thought, back when American Pie kickstarted five careers, they'd be 13 years in the same roles: sensitive Jim (Jason Biggs), insensitive Stifler (Seann William Scott), brawny Oz (Chris Klein), brainy Finch (Eddie Kaye Thomas) and Nicholas's straight man, Kevin. These names, Thomas says, "will forever be stamped on our foreheads."

"It told a relatable story about guys losing their virginity," says Biggs, explaining the 1999 film's success. It did, but beneath that there was a moving story about school friends bracing to leave one another behind for college. "It was about the end of innocence," says Thomas.

The new film catches up with the group as dissatisfied adults. In one scene they wince at the cheerful predictions they'd made for themselves in a school yearbook. "The first movie was about trying to cross the threshold into adulthood, how weirdly tragic that is," says Thomas. "Well, there's also something tragic about the same guys coming together at 30 and thinking: I'm not who I thought I was gonna be."

Arguably only Scott has found critically ticked work away from the franchise; his 2008 film The Promotion has become legendary among indie comedy fans. Biggs took a job this year as the voice of a Teenage Mutant Ninja Turtle for Nickelodeon. Nicholas has put out three CDs of acoustic music.

"I've seen every film these guys have been in," Nicholas says, gamely, but Thomas, cheekier, soon starts to tease the others about certain wobbly career choices. "Wedding Daze," he sings at Biggs, a reference to a creaky rom-com. "And, hey, I loved Bulletproof Monk," he says, for Scott's benefit.

"I'm very thankful for the transition that American Pie created for me," says Nicholas, once a child actor and using his industry savvy to tug the group's conversation back on-message. "It helped turn me from a kid actor [in films like the 1993 baseball fantasy Rookie of the Year] into having a career as an adult." But, no, the gang aren't going along with this. Biggs quickly interrupts: "A career which included making out with my wife in a movie."

"Yes it did," says Nicholas, wincing. (He starred with the actor Jenny Mollen, now married to Biggs, in a National Lampoon film called Cattle Call.) "In fairness it was before Biggs knew her," he says. "That's kind of hot," puts in Thomas. "That's actually really hot," says Klein.

Since the last formal Pie sequel – 2003's American Pie: The Wedding – there have been a series of cheaper spinoff films made under the brand umbrella but with different actors, poorer returns. Was the original cast's return to the franchise an effort to reclaim it, somehow, from straight-to-DVD territory? Scott says he wasn't totally aware the spinoffs were even being made. Thomas says: "I mean, listen, we saw some of those films and they were funny. But I never felt like we were not associated with American Pie. I never felt the need to remind people that, you know, we were the guys." There is a rueful chuckle of agreement from the others. You'd assume, over the last 13 years, there have been plenty of instances of being called Stifler and Finch in real life; of having people shout out catchphrases from the films ("Hey, shit-break!") "I've played this character four times," says Scott. "I've done variations of the character that are not as funny. But I'm surprised sometimes how people can't differentiate between reality and what's on screen."

They're all in their 30s. Might there come a time when they're just too old for the franchise's famously gross humour? "Apparently not," says Thomas, while Biggs says, "You kind of become anaesthetised to it. During filming you're removed from it sensually and it's only later you think: Oh God, I was, I dunno, wearing a gimp costume."

The group agree this was a highlight of shooting the new sequel. Biggs had to dress in a leather bondage outfit – pants, dogcollar, leash – as part of a setup in which his evening of sexy role-play abruptly segues into a public fist fight and an arrest. "Biggs was in the gimp outfit for four days on set," says Thomas. "We'd be shooting late at night, talking about baseball, and then you'd realise, yep, he's still in leather pants with a built-in plastic ball cup." "Just another day," sighs Biggs.

Will there be more Pies? "I plead the fifth," says Biggs, before Nicholas points out that the fifth amendment doesn't factor in the UK. "He was pleading for a fifth American Pie," says Thomas. "For the love of God," he says, while the others crack up, "make another sequel!"

American Pie: Reunion is out on Wednesday 2 May