Josh Peck has a motto: “Drop the robe, and hope that when it falls the world doesn’t fall with it.” Admittedly, it’s not that catchy, but it was an immeasurable help to the 21-year-old former teen TV star (of the hugely successful Drake & Josh) during his first screen sex scene, in the New York-set coming of age movie The Wackness.

Here, as an amiable marijuana dealer, he trades blows and dime bags with his substance-addicted psychiatrist, Dr Squires (played by Ben Kingsley), he smooth-talks the local criminal kingpin Percy (Method Man, from the Wu-Tang Clan), and ultimately makes sweet whoopee with Dr Squires’s sassy daughter Stephanie (Olivia Thirlby).

“When I was told I had got the movie I immediately regressed into a neurotic psychosis about the fact that I was going to be butt-naked on screen,” explains the Los Angeles-based actor, who’s currently the toast of the independent movie scene with forthcoming roles in the quirky dramas American Primitive and Safety Glass (again opposite Thirlby). “I usually leave my sexual self-exploration behind closed doors. For it to be projected on screen was petrifying.”

Peck’s discomfort with his own body is not just actorly vanity. For until very recently this poster-boy beauty was, well, fat. How fat? He lost a massive 120lb in two years without dying. Before that he was successful, yes, but in roles that required blubber first, talent second. In Drake & Josh he carried on the Laurel & Hardy tradition by playing the fastidious fat friend to the über-skinny wildcard, Drake Bell. In the hipster drugs movie Spun he was simply cast as Fat Boy, while in the teen drama The Newcomers he played, ho-ho, Slim.

Even in his most prestigious preWacknessrole, as the mercurial bully in Mean Creek, his weight there was key, and part of his perceived repugnance. “ Mean Creek was a definite crossroads for me,” he says. “Because I realised then that I didn’t want to wait around for another part that had integrity. And neither did I want to play the chubby best friend forever.”

Thus, through what he describes as a “boring regime of exercise and diet” he slowly began to drop his excess baggage. He says, however, that despite his newly rebuffed form he still bears the lifelong scars of being the taunted, ridiculed, fat kid. “The wounds are so deeply embedded in me, man,” he says, sighing at the thought of his own pain. “It was such a constant during so much of my formative years that even now, when my exterior matches something that might not get ridiculed, it’s a constant challenge, every day, to reshape my way of thinking.”

Raised by a single mother in a one-bedroom apartment in the tough Hell’s Kitchen area of New York, he says that he has no idea why he became a heavy child. “I dunno,” he says, hazarding a guess. “Maybe because food tastes good and treadmills suck.”

What he does know, though, is that he parlayed his pain into jokes and gags, and, at the age of 8, was onstage in Manhattan comedy clubs, making a reputation as a precocious talent. “The comedy came easy to me,” he says. “If I hadn’t directed it into comedy clubs, it would’ve brought me into the principal’s office. Because that’s where I was most days.”

He adds that he remembers that era of pre-Giuliani New York with a certain self-confessed idealism. “I’d walk to school in the morning, between 10th and 9th Avenues, and I’d meet these lovely, provocatively dressed women who’d make sure that I was OK and that I had a good lunch. I had no idea that they were prostitutes. They were just caring young women to me.”

Eventually, impressed by his burgeoning showbiz smarts, Peck’s mother Barbara moved with him to LA when he was 14. There, bit-parts in ER led to a co-starring role in The Amanda Bynes Show and finally a starring role in Drake & Josh.

He says that the move to LA was not just about his career; it was always deeper than that. “Even today my mother says, ‘I never moved here with the hopes of you becoming famous. I just knew that performing gave you confidence. What more could a mother want for her son?’ ”

He says that confidence still eludes him, that keeping the weight off is a struggle, and that he’ll follow three days of obsessive fitness with a relapse into marshmallow cereal. He’s single, and says that having girlfriends is a complicated process for him, especially with the voice of his inner fatty taunting him. “If a girl likes me, I’m like, ‘What? Is this girl crazy? How could she like someone that looks like me?”

He says that acting opposite “Sir Ben” was the highlight of his career so far, and included many strange moments. “There’s one scene where I’m sitting there, witnessing Method Man, a member of the Wu-Tang Clan, pass a joint to Ghandi,” he says. “And I’m thinking: ‘This is a pretty surreal experience.’”

He adds, naturally, that Kingsley was an inspiration to the entire cast and crew, and a tireless example of someone who has managed a successful career with seeming ease and grace. “But, for me, it’s not all about that,” he says, on reflection. “My best day is when I hang out with my Mom and my dog and never once think about my career.”

The Wackness is released tomorrow

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