The prolific writing trio behind Peep Show and The Thick Of It tell why they're pushing the mainstream so close to the edge

Yet chatting over a rather more palatable lunch in the smart, upstairs surroundings of a central London restaurant, Sam Bain, Jesse Armstrong and Simon Blackwell admit that they were "terrified" at the thought of 250 Glaswegians sitting stony-faced through recordings of The Old Guys, their new Friday night sitcom for BBC One. More intimidated even than when they met Sopranos star James Gandolfini.

"I was in awe," admits Blackwell. "This great hairy man."

"He's not someone you meet and think, 'Oh my God, you're so not like your character,'" Armstrong concurs.

In The Loop, starring Gandolfini, Tom Hollander and most of The Thick Of It cast premiered at the Sundance Film Festival on Thursday, and will open Glasgow Film Festival on February 12. Directed by Armando Iannucci and scripted by the Glaswegian with Armstrong, Blackwell and Tony Roche, the cinematic spin-off focuses on politicians and bureaucrats in Britain and the US scrabbling around in the build-up to war, with petty rivalries and cynical career manoeuvring overshadowing an almost incidental conflict. Scheduled for release in the UK this spring, one critic has already announced that "it might even be the best British film of the year".

As is their habit, the writers were on set throughout the shoot and former Labour researcher and New Statesman columnist Armstrong recalls struggling to focus as Peter Capaldi, playing temperamental spin doctor Malcolm Tucker, and fellow Scot Paul Higgins as his psychotic lieutenant, prepared themselves in rooms on either side of him.

"As he often does, Peter started singing Sinatra to himself," he says. "And Paul was repeating some of Simon's lines which didn't make it into the film: 'I've got a puppy f***ing machine, puppy f***ing, puppy f***ing…' Slightly distracting when you're trying to come up with new material."

Meanwhile, Four Lions, the working title for the jihadist comedy penned by Armstrong and Bain for Chris Morris begins production this summer for a cinematic release later in the year. The pair remain tight-lipped about the extent to which Morris – whose Brass Eye paedophile special caused such a tabloid furore in 2001 and who has spent three years interviewing terrorism experts, police, the secret services and imams, as well as ordinary Muslims – has adapted their initial script, aside from enthusing that "his level of research was amazing" and "it's incredibly exciting".

Blackwell though, who has seen a copy purporting to show "the Dad's Army side of terrorism", describes it as "very funny. It hits precisely the right tone."

So why then are they so concerned about The Old Guys? Especially as it stars sitcom veterans Roger Lloyd Pack (Trigger in Only Fools And Horses) and Clive Swift (Richard in Keeping Up Appearances) as Tom and Roy, growing old disgracefully while lusting after their neighbour Sally, played by Jane Asher. The IT Crowd's Katherine Parkinson completes the cast as Tom's daughter Amber.

"Having that citizens' jury out there is scary," says Armstrong of the trio's first self-originated studio sitcom, recorded at BBC Scotland in Glasgow. "I'd be lying if I said I didn't watch it and think, 'Bloody hell. Could this work without a laughter track? Could we be more subtle?' But it's the show we've always wanted to write."

According to Bain, "with Peep Show, a sitcom only watched by a million or so people, we're protected by a sort of layer of cool and the fact that visually, it's shot in an interesting (point-of-view] way. With a more conventional show like The Old Guys, the audience have got to invest in the characters immediately. Even though there's nothing stylistically radical about Frasier or Seinfeld, they feel sophisticated because the characters are interesting. That's our aspiration too."

Blackwell expands upon the pitfalls. "I've done a lot of gag and sketch writing in front of a live audience," he says. "But never a narrative. The temptation is to just fill it with as many jokes as possible because you want that constant laughter. The danger then is that it becomes like a stand-up routine and you don't get a satisfying story."

The trio first collaborated on a failed pilot for the late producer Harry Thompson, whose mantle for overseeing cutting-edge comedy at the BBC has arguably now passed to Iannucci. Blackwell, who was handed his first break on Iannucci's Radio 4 show Weekending, went on to be a joke writer under Thompson's tutelage on Have I Got News For You. After he'd worked with Armstrong on The Thick Of It, he and Bain, who met at Manchester University, enlisted Blackwell's help when their workload became too onerous to complete all six episodes of Peep Show's last series.

Easily inhabiting the self-absorbed mindset of dysfunctional flatmates Mark and Jeremy, played by David Mitchell and Robert Webb, Blackwell remains a useful ally says Bain, because being Oxford-based, he "doesn't understand London rates of pay". Moreover, having introduced a gun and the spectre of male rape into their sitcom too, Armstrong is keen to stress that "Simon takes all the credit for that particular episode".

Although The Old Guys was conceived without a specific channel in mind, and actually pre-dates Peep Show, with Bain and Armstrong coming up with the idea in 1999, they acknowledge that Iannucci and The IT Crowd writer Graham Linehan put a "certain amount of friendly pressure" on them to try writing for a more mainstream audience.

"I genuinely don't think we've made any concessions though," says Armstrong.

"We took out some swear words," Bain interjects, "but that was because it sounded wrong in the actors' voices."

"It sounded like we were trying to get a laugh from making your granny say 'f***'," Armstrong concedes, before adding: "We never had a cast in mind, though. But we did feel, 'wouldn't it be fun to write for a generation with loads of brilliantly talented comic actors?' We reckoned that if we wrote for that age group, we might be able to punch above our weight, get somebody really amazing. And that's how it turned out with Roger and Clive."

Despite both Peep Show and The Old Guys having been provisionally titled 'All Day Breakfast' at different times in their development and focusing upon the domestic setting of two bachelors, Armstrong reckons that the pairings "complement and rub up against each in other in different ways".

"One of the fun things about writing comedy is that you can actually forget about their age to an extent," Bain says. "You shouldn't think 'right, what would an old person do?'"

"You'll only end up writing all your jokes about colostomy bags," Armstrong says.

Nevertheless, despite a greater tendency towards farce than their Channel 4 sitcom, some storylines in The Old Guys, such as Tom contemplating visiting a prostitute or embarking upon a civil partnership of convenience with Roy seem exceptionally edgy for the BBC's flagship channel. And one episode, involving the death of a supporting character, is remarkably dark.

"We were slightly worried about whether the audience would feel they were allowed to laugh there," says Blackwell. "Thankfully, big relief laughs followed those early nervous chuckles. Hopefully, in every episode there are moments where you think, 'I wouldn't expect this in a BBC studio sitcom.' Not because it's gratuitously edgy but because it's emotionally interesting."

Producing the show is Absolutely alumni Jack Docherty. "He's a comedy hero of ours, though we'd never tell him," says Blackwell, noting that "there's an awful lot of exciting comedy coming out of Scotland at the moment, it's like Naked Video and Absolutely in the Eighties."

As well as the sixth series of Peep Show this summer, Bain is working on a "relentless" one-act play "because he hates ice cream", Blackwell has contributed to ITV's forthcoming call centre comedy Mumbai Calling and Armstrong plans to write a film about Rupert Murdoch for Channel 4, recreating "events that haven't happened yet at a future Murdoch family gathering. Perhaps I'll have an idea what it's like after I've not written it and the legal team have not okayed it." v

The Old Guys, BBC1, Saturday, 9.30pm. In The Loop opens Glasgow Film Festival, February 12 www.glasgowfilmfestival.co.uk