When Harry Potter fans learned last summer that Rupert Grint and Emma Watson had filmed their long-awaited kiss as Ron and Hermione, they went bonkers – don't type "Ron+Hermione+kiss" into Google Images if you want to keep your breakfast down. Quite how they will contain themselves watching Cherrybomb, Grint's new, non-Potter film is unclear. Within half an hour he has not only wrapped his chops around a busty bottle-blonde, but has received what looks to be an extremely enjoyable round of fellatio.

Sitting on the roof terrace of a London hotel, Grint, 21, squirms at the memory of it. How did he go about practising that face? "It's really hard," he says. "It's very hard to lose yourself in the moment and just kind of get absorbed by it because there's always going to be a big fat guy nearby holding a boom."

Cherrybomb is a Skins-like romp about two Belfast teens vying for the attentions of the same girl. Grint plays Malachy, the nice boy of the piece, who nevertheless snorts coke and swears like a squaddie. So is he using Cherrybomb to prove he is more than just a ginger boy wizard? "I suppose kind of it does say that, but it wasn't a conscious decision," he says. "It did attract me that it was a much different, adult kind of role." Was he trying to shock? "I suppose, yeah. I don't know. I mean, it's going to happen. I'm not 11 any more, so the films I am doing are going to match my age."

Grint, of course, was just 11 when he was chosen to play Harry Potter's best friend. By the time the shooting of the second part of the Deathly Hallows wraps up in a few weeks' time, he will have been famous for half of his life. It is unsurprising, then, that Grint seems both older than his years and also young for his age. He is the world's third highest-earning actor aged 21 or under, according to Forbes magazine – with only Radcliffe and Watson ahead of him – but he still lives at home with his parents, younger brother and three little sisters. Until five years ago, when he had already earned millions, he was sharing a bedroom. He now has his own room but no plans to move out. How come? "Because it's really easy, really. It's right by the studios. I get on with my family. My washing is done. It's just nice. I probably will eventually move out," he says with a languid air that suggests he is in no hurry whatsoever.

Grint's unusual adolescence has led to him being far less independent than his contemporaries. He claims to have never cooked anything in his life – "But I can heat things up" – and wouldn't know where to begin if he had to run his own life. Such as? "Like … getting a train. I've never done that on my own. I wouldn't know how to." He thinks being in Harry Potter has arrested his development more than accelerated it: "Everyone does everything for you. There's always someone there to organise parts of your life."

Endearingly, Grint says he didn't really grasp what he was getting into when filming began on the first Potter film. "I remember being quite surprised when they said I was going to get paid for it, so [money] has never really been much of a motive," he says. "I never really knew who I was working with either – like Alan Rickman, all these huge people who I really respect now. Back then I thought they were just some old people."

He seems happy leading his slightly closeted existence, never getting on trains and having his mum cook his tea, though he does venture out when he fancies it, despite the hassle it can bring. His devotees constantly send him presents, and there is one particularly time-rich woman in Brazil who sends him oil paintings depicting scenarios from his films, both real and imagined. "A lot of them are love scenes with Emma," he says. It can only be a matter of time before he gets that blowjob scene through the letterbox. Let's just hope his mum doesn't open his post.