'I have 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 schoolboys in The Inbetweeners, he 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 recent 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 such as "clunge", "bus-w**ker" 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 British comedy ever. It took more than £45m at the box office and later sold 1.3 million DVDs in its first week of release – not bad for a 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 is 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," he says.

Now, though, comes the task of unblurring the boundary between actor and character. The Greenwich-born 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 has been in The Increasingly Poor Decisions of Todd Margaret, a curious US-British sitcom starring David Cross and Sharon Horgan. Harrison played Cross's street-savvy sidekick Dave, who, in series two, turned out to be an aristocrat. Jon Hamm (Mad Men's Don Draper) played 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 the promising young writer Rob Hayes, which opens 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 may be drawing on personal experiences of addicts, Harrison does not 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," he explains. "The drink was a catalyst but he always had the potential to do horrible things."

Despite admitting to nerves, Harrison is 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 know everything about, up to this week."

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

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