Jonny Sweet: smell of success

Timing. It's the secret of comedy. And Jonny Sweet couldn't have timed his television debut in a starring role any better. At the end of 2009, the comedian/actor played a floppy-haired, youthful David Cameron in When Boris Met Dave. The More 4 docu-drama about the two Tories' Bullingdon Club days screened on the eve of Cameron's last party conference speech as opposition leader. A few months later, Cameron was Prime Minister, and Sweet, 25, had his selling point for life – whether he liked it or not.

"I'm beginning to wonder if I'm being typecast. Certainly since David Cameron I get a lot of offers to play posh pricks," says Sweet. "It's quite good to have a thing when you're starting out, I guess. And it's not like I could play a Mancunian miner."

I think he's probably doing himself a disservice, but for now at least Sweet, with his baby face, swoop of hair and plummy drawl seemingly dredged up from another era, has the market in comedy toffs cornered. His debut solo show, Mostly About Arthur, a Bunterish tribute to his (fictional) dead brother, won the Edinburgh Comedy Award for Best Newcomer while Party, in which he played a posh-but-dim student hatching a political movement in his daddy's deluxe shed, transferred to the West End. His follow-up Fringe show, Let's All Just Have Some Fun (And Learn Something, for Once), now on at London's Soho Theatre, develops his buffoonish persona into a hapless military historian in a too-tight pink shirt, delivering a bogus lecture on HMS Nottingham.

His next role will reunite him with his writing partners Simon Bird and Joe Thomas, better known as Will and Simon of the E4 smash-hit comedy The Inbetweeners. Was he too posh to join them at Rudge Park Comprehensive? "Maybe. But there are so many things that happen in a casting process. Looking at it now, I don't know which part I would have played." In Chickens, a new Channel 4 sitcom set on the home front in the First World War, they play the only three men who don't go to war "in a village full of women who hate them for it." One is a patriotic but injured soldier, one a conscientious objector, "and the other is just a posh coward – that's who I play," says Sweet.

Chickens is the culmination of the trio's working relationship, which began in Freshers' Week at Cambridge. Sweet met Thomas on his first night at Pembroke College and was persuaded to try The Virgin Smoker, a student night for first-time stand-ups. Together they performed a sketch about a man who pretends that he works at a sea life centre in order to impress an old schoolfriend. "It ended really sadly, with Radiohead's 'No Surprises' coming over the speakers," recalls Sweet. "It went down quite well. Joe was really nonchalant about it. I was excited, in quite a girlish way." They met Bird there and the three became Footlights regulars before taking their own sketch show up to the Fringe in 2007.

House of Windsor, featuring a delusional Tim Henman who thought he'd won Wimbledon and Legoland's middle-management, among others, marked the trio out as fresh faces with a penchant for skewering the stilted manners of the middle-classes. The following year, they staged a site-specific piece in an office, inviting audiences to sit in on the tedious board meeting of a fictional business. "Some people would walk in with their four pints of beer, expecting stand-up, look around and walk straight back out again." The same year they performed The Jonny and Joe Show, a deliberately hit-and-miss sketch show in which the main joke was the pair's incompetence as performers. "We were very forgetful and kept doing the same sketch, over and over," says Sweet. "Some people really went for it. Some people got quite angry. One guy stormed out and I chased him out of the auditorium. As a three we've always been attracted to that sort of thing."

"That sort of thing" is a kind of edge-of-your-seat discomfort that comes not, as so often in stand-up, from ranting about taboo topics, but from a malaise on the part of the performers whose polite demeanour disguises men teetering on the brink. "That's the only thing we've ever talked about – doing something different. We would often sacrifice laughs for that when we were getting started," says Sweet. Unsurprisingly, cringe comedies I'm Alan Partridge and The Office ("it was too influential on me, really. I had to wean myself off the mannerisms") were formative inspirations.

When Bird and Thomas took off in The Inbetweeners, Sweet struck out alone. His solo shows are similarly awkward affairs, dedicated to purposefully uninteresting subjects. The hero of Mostly About Arthur is a "blurbist", or someone who writes the bumf on the back of books. "I just liked the general argument of a show being to save the legacy of someone who is obviously terrible," explains Sweet. His Powerpoint-assisted (or hindered) presentation on HMS Nottingham is a celebration of "one of the biggest non-events of military history". It's all delivered with an undertow of chaos and a manic glint in his eye – the same unpredictable brilliance that saw Sweet singled out from a cast including Alistair McGowan and Hugh Dennis, as the one who "came closest to catching the anarchic 1960s spirit" of Peter Cook in BBC2's Pete and Dud: The Lost Sketches.

While some of his live show, "the seat-of-the-pants stuff", isn't scripted, it's a carefully planned exercise in probing boundaries. Last year, he welcomed audiences in with a stiff hug. This year he was a little more intrusive, treating them to affectionate hair-ruffling and chummy nose-tweaking. At one point, his producer had to take him aside and ask him to tone it down. "He said I'd never get over the hump of awkwardness I'd created for myself. It's a case of managing those moments." More endearing than edgy, he largely gets away with it – but not always; last summer, one irate punter started a fight with him on stage (and later came to his dressing room to apologise). "Really, I'm the least likely person to get into a fight," says Sweet. "I do find myself doing things on stage that I wouldn't do in real life. Occasionally that has created dicey situations. I haven't the composure of Stewart Lee, or real comedians."

Sweet dislikes performing live, as a rule. "I'm usually genuinely ill with fear," he says. "A lot of my peers don't know why I do it. My characters come out of me being terrified on stage. There are people out there who are expecting me to be funny – that notion is just awful to me, terrible." He'd rather be writing – which he does most days, from 9 to 5, in the British Library. "I'm not a stand-up and I don't really aspire to being a stand-up. I like creating little worlds and stories."

Having played a narrator in Under Milk Wood and given a "nominally comic" speech in assembly at school in Nottingham, Cambridge provided his first real taste of the stage. The previous Footlights generation – Tim Key, Mark Watson, Tom Basden, Stefan Golaszewski – were just graduating, with flying colours. "I basically thought, 'these people are rock stars. I want to be like them'." Sweet has since worked with his heroes, appearing in Basden's Party alongside Key. Together with Nick Mohammed, John-Luke Roberts and Anna & Katy, he's part of a new wave of exciting, low-key comedians to emerge from the university in the last five years.

Having devoted the first two years of his English degree to Footlights, Sweet eventually buckled down, graduating with a First. Though his mother hoped that he might follow his father into the legal profession, Sweet instead took an internship at Endemol, where he worked for nine months coming up with television concepts. One evening, he skived off the filming of Supersize vs Superskinny to perform his House of Windsor show for the first time. When the lights went up, his boss was sitting on the front row. "It was one of the worst nights of my life. He was so nice about it, he could have sacked me."

Playing the future Prime Minister – "they were holding out for Daniel Radcliffe," he claims, improbably – was his big break, television-wise, though it didn't please his left-wing family. "My Dad, who obviously thinks I'm a more charming actor than I am, said, 'if you're responsible for getting Cameron into power, I'll disown you'." In his audition he had to improvise the politician's Oxford interview. "I was asked about social cohesion and my views on it. He's famously quite an opaque person – impressionists have a difficult time of it. I did it as sort of a sterner version of myself. Perhaps too stern. I looked quite Silence of the Lambs in some of my glances."

Chickens begins shooting in May and Sweet is writing "a sort of textbook of modern parlour games" to accompany his current show. Plays, sitcoms, rom-coms, he'd like to tackle them all. "I just want to keep plugging away. Realistically, writing a sitcom as good as Peep Show, you can't expect that to happen – it's so brilliant." If anyone can take on the challenge of writing the next great British sitcom, though, Sweet and his gang can.

'Let's All Just Have Some Fun (and Learn Something, for Once)', Soho Theatre, London W1 ( www.sohotheatre.com; 0207 478 0100) to 26 January