Blake Harrison: 'I've never had a plan B'

For a significant portion of the past four years, Blake Harrison has worn an expression of gormless confusion. As Neil Sutherland, the dimmest of the four Inbetweeners, he has perfected a slack-jawed, dead-eyed gawp that suggests cogs creaking into place one tooth at a time. A dial-up modem in a broadband world, Neil's reactions come with a time-delay and his questions - "How much Lego can you stick up your bum?" - come from nowhere.

Playing stupid gets laughs, but it can result in the curse of typecasting; every actor's recurring nightmare.

Certainly, it's a worry for the 27-year-old actor, whose thick black stubble seems designed to distance himself from his teenage counterpart. "If I was offered dumb roles for the next 20 or 30 years, I wouldn't be satisfied at all. I've already turned several down, because it's the last thing I need to do. Is there any way that it's going to be better than The Inbetweeners?"

He's probably right. No sitcom in the past few years has had the same impact as The Inbetweeners. It follows four sixth-form social misfits through their romantic mishaps, alcoholic excesses and inopportune bowel movements. Their colourful vocabulary - words like "clunge", "bus-wanker" and "fwiend" - has entered the teenage lexicon and, over three series, it has built a huge fanbase. So much so that last year's spin-off film became the highest-grossing UK comedy ever. It took more than £45m at the box office and later sold 1.3m DVDs in its first week of release. Not bad for a British film that cost £3.5m.

"We expected the film to do alright, because we knew we had a following," says Harrison, genuinely wide-eyed at the scale of it all, "No one thought it would do as well as that. And none of us expected it to do so well critically."

I ask why he thinks The Inbetweeners has been so successful. "The writing," he says, almost before the question's fully out. "If you were to rank comedies, pound for pound, in terms of laughs, I really believe The Inbetweeners is right up there. It doesn't do anything except try to make you laugh. Also, people relate to it incredibly easily. It's rare to have a sitcom that's nostalgic and current at the same time. I don't think the writers Iain [Morris] and Damon [Beesley] get enough credit for that."

The film was meant to be the final outing for the gang, but the extent of its success has made the question of a follow-up inevitable. As yet, however, there are no plans - concrete or otherwise - and Harrison insists the decision rests with Morris and Beesley. "If they were to write it, I think we'd be up for it, because we trust their writing so much."

Now, though, comes the task of unblurring the boundary between actor and character. Harrison has had a variety of guest roles in sitcoms including Him & Her, White Van Man and the Dickens spoof The Bleak Old Shop of Stuff, but his only other lead so far came in The Increasingly Poor Decisions of Todd Margaret, a curious American-British sitcom starring David Cross and Sharon Horgan. Harrison plays Cross's street-savvy sidekick, Dave, who in the second series, turns out to be an aristocrat. Jon Hamm (Mad Men's Don Draper) plays his butler. "It's got such cracking comedic actors in it. The only problem with Todd Margaret is that not many people in this country have seen it."

Harrison's concerns about typecasting are also partly responsible for his forthcoming West End debut in Step 9 (of 12), a darkly comic play from promising young writer Rob Hayes opening this week. Feisty and full of black humour, its laughs leave a very different aftertaste to The Inbetweeners: bitter and dry. Harrison plays Keith, a recovering alcoholic, who is (as the title suggests) three-quarters of the way through the Alcoholics Anonymous 12-Step Program. That particular phase involves apologising to those you've hurt along the way. First up are his foster parents.

"Keith sees things in black and white. With the ninth step, they specifically say, 'If you're going to hurt someone by bringing up the past, don't do it.' Keith completely disregards that. He thinks, 'I'm on step nine. I need to get to step 10.'"

While he's drawing on personal experiences of addicts, Harrison doesn't see the play as one about addiction. The causes run deeper. "The things that have really affected Keith's life happened before he even had a sip of alcohol. The drink was a catalyst, but he always had the potential to do horrible things."

Despite admitting to nerves, he's relishing the chance to delve into a dark character with a violent past, not only as an opportunity to prove his range to any sceptics, but also for the extra creativity that theatre offers over television. "There's so much more room to create what you want to create," he enthuses, "It's not just a case of what are these characters, who we already know everything about, up to this week."

It's something he's been hoping to do for a while, and he recently auditioned to replace James Corden in One Man, Two Guvnors, but was deemed "too skinny" for the role. He was last on-stage four years ago, in a pub theatre in Kennington just after leaving drama school. At the time, he didn't have an agent and was earning money by "sticking the labels on the inside of buses between 8pm and 4am."

"I've never had a plan B, really, which I look back on as the scariest thing in the world."

Harrison attended the Brit school aged 14, following a stint as a workhouse boy in the West End musical Oliver! Intending to go into musical theatre, he studied drama and dance; subjects that brought extracurricular challenges. "My mates were constantly ribbing me for doing dance. As a 14-year-old lad from a working-class background, there are certain stigmas. I remember my Dad being like, 'Dance? You big sissy.' Peckham's not an easy place to grow up in, especially as a kid that wants to sing and dance."

Sport was prized over the the arts, but for all his enthusiasm, it was never Harrison's strong suit. Nowadays, he plays a bit of amateur football. "I'd come off the bench for the last 20 minutes and the shout would always go up, 'Oi, mark Neil.'"

The struggle to shake off the character is ironic. When he first auditioned for The Inbetweeners in 2008, then using the working title "Baggy Trousers", Harrison ignored the role of Neil because he didn't think he suited it.

"I looked at the lines and thought the one part I won't get is Neil. I'll probably get Simon, the hopeless romantic, so I focused on that. As soon as I got into the room, the casting director said: 'You're reading Neil.' I thought, 'This has gone horribly.'"

'Step 9 (of 12)', Trafalgar Studios, London SW1 (0844 871 7627) to 26 May

All grown up: What are the rest up to now?

Simon Bird

His role as "briefcase wanker" Will McKenzie won him two individual British Comedy awards. A former Footlights president, Bird has since hosted the anarchic BBC3 panel show 'The King is Dead'. He's just finished filming Michael Winterbottom's 'The King of Soho' with Steve Coogan and is about to start on the second series of Channel 4's 'Friday Night Dinner'.

Joe Thomas

After embodying Simon Cooper, he played another hopeless romantic - Kingsley in 'Fresh Meat', the award-winning sitcom about a student house from the writers of 'Peep Show'. Currently writing and starring in 'Chickens', a comedy about men left in England during the First World War, with Bird and comedy partner Jonny Sweet.

James Buckley

The youngest of the 'Inbetweeners' cast played the foul-mouthed fantasist Jay Cartwright. He was also the young Del Boy in the BBC's 'Only Fools and Horses' prequel 'Rock & Chips', and had a role in Comic Strip's 'The Hunt for Tony Blair' last year.

Emily Head

The actress who played Carli D'Amato, the object of Simon's affections, is a Brit-school graduate - and daughter of Anthony Head. She has since turned to theatre, starring in 'An Inspector Calls' around the UK and warped romcom 'Third Floor', at London's Trafalgar Studios last year.