Rupert Grint's magic world of hex, drugs and rock'n'roll

The Harry Potter star has grown up and even gets naked for a movie about a hedonistic Belfast youth. But he'll still miss the role that made him famous, he tells James Mottram

It's just before 6pm and Rupert Grint has finished for the day. I feel like saying "Hard day at the office?" But I don't. After all, the red-haired one from Harry Potter must be feeling it right now. He's been filming the final instalment of the hit franchise based on the J K Rowling novels for just over a year now. "We haven't got long left," he says. "Just the final parts of the second part of the film." Not that exhaustion has set in yet. Rather, a feeling of uncertainty has enveloped him. "It'll be strange saying goodbye," he says.

With Harry Potter and the Deathly Hallows split into two films (the first due out in November, the second in July 2011), it will be some time yet before we bid farewell to Grint's character, Ron Weasley, and all the other Hogwarts pupils. But for the 21-year-old, a life-changing experience that began half his life ago is due to end this June when filming finishes. "It's a weird feeling actually," he admits. "I never really thought it would end. I never really saw this day coming."

In truth, I'm expecting to find a rather nervous figure before me. It can't be easy facing the prospect of unemployment for the first time. While most actors are hardened to it, Grint, and his co-stars Daniel Radcliffe and Emma Watson have probably been institutionalised by their time at Leavesden Film Studios, the former Rolls-Royce factory on the outskirts of Watford that has been the home to Harry Potter since shooting began in 2000. "I don't know if I'm good enough to have a long career," he told one interviewer last year. "I've got a bit of an inferiority complex about my acting. My self-esteem is quite low in that sense."

Yet judging by his reaction today – and proving that Radcliffe was correct when he described Grint as "the most totally laid-back person you'll ever meet" – he's changed his tune. Quietly self-assured, while still as modest as the ripped jeans, T-shirt and dirty red- and-white-striped Converse boots he wears suggests, he simply shrugs when asked if he's worried about his post-Potter future. "I've loved every minute of Harry Potter," he says. "Yes, it'll be quite sad to see it go. But I'm also looking forward to being a bit more free and seeing what else comes along."

Admittedly, with estimates putting his wealth in the region of £9 million, such a safety net must help soften the blow. But there's more than money to consider in what must be akin to the feeling of leaving home for good. "It's been such a tight crew since the first film," he says. "Not many people have changed. It's a real family atmosphere. And the place as well... Watford in general really. I've spent more time there than anywhere. I don't know. It'll be weird not going there every day." He considers this for a second, then laughs. "I'll probably get over it."

Like his co-stars, Grint has already started making preparations for his departure. As far back as 2002, he featured in children's tale Thunderpants, and has since appeared alongside his Potter co-star Julie Walters (who plays Ron's mother) in the coming-of-age comedy Driving Lessons, "the first grown-up thing I'd ever really done", as he puts it. But while that saw him portray a shy teenager not a million miles from himself, his latest film, Cherrybomb is something else. "It was quite scary. It felt like a massive step. Filming in a different country, with a different accent, a crew I didn't know... it was a little bit daunting."

Directed by Lisa Barros D'Sa and Glenn Leyburn, Cherrybomb is a Belfast-set drama with lashings of sex, drugs and drink that might raise a few eyebrows among the Potter faithful. "It wasn't a conscious thing to do something completely controversial and shock people," argue Grint, who plays Malachy, a straight-A 16-year-old who works on reception at a leisure complex. While it may not be an episode of Skins, it's still a valid attempt to portray teen life realistically, as Malachy and his wild mate Luke (Robert Sheehan) find themselves competing for the affections of new-girl-on-the-block Michelle (Kimberley Nixon).

Playing the nerdy sidekick to the more charismatic hunk may be something he's already used to thanks to Potter, but Cherrybomb does boast it's fair share of sex scenes between Grint and Nixon, who came to prominence in the 2008 Noël Coward adaptation, Easy Virtue. It's certainly a far cry from the rather chaste kiss he shared with Jessie Cave, who played Lavender Brown in Harry Potter and the Half-Blood Prince. "That was just a kiss, really. It was suggestive more than anything," he says. "This was a lot more intimate. It was quite nerve-wracking. I was quite nervous about it."

While he does get his top off, Grint admits he's uncertain whether he could echo what Radcliffe did on stage for Equus and go full frontal. "I don't think I could. Just being on stage would be quite a scary thing, let alone with no clothes on. It takes a lot of courage." Yet he feels Potter obsessives will accept him in this more mature role. "I suppose, as I'm getting older, the fans are getting older with me." Has he ever fought with a mate over a girl, like Malachy, I wonder? "I never really did. I haven't had the chance!" Comments like this make you realise how curious it must be to experience your formative years growing up on a film set.

But if Grint hasn't spent his youth chasing girls, he's as grounded as they come. Born in Harlow, he still lives in Hertfordshire, where he was raised and began acting in school plays. Preferring a round of golf to a night out on the tiles, he doesn't come across as a movie brat on a path to self-destruction. Frankly, he doesn't have the constitution for it. "I suffer really badly from hangovers," he says. "I need two days to recover." He's even avoided that most distressing of stigmas – playground teasing over being ginger-haired. "When you're at school, people call you 'ginge' and that. But it's never been anything nasty. I know some gingers get a hard time over it. But I'm pretty grateful for it!"

Another reason he's arguably grateful for his red hair is that it hasn't quite turned him into a teen sensation. While his face doubtless adorns many a teenage girl's wall, he doesn't really suffer from the hysteria that greets Twilight star Robert Pattinson – just two years older than Grint – wherever he goes. "I get recognised occasionally but nothing like that," he says. "It's crazy. You just can't really go anywhere. I've got a much more manageable existence. Must be pretty..." He stops for a second, imagining such an intrusion into his life. "It's just come from nowhere [for him]. It's such a quick thing. Good luck to him."

The eldest of five, Grint's equilibrium evidently stems from his upbringing in a strong family unit. "We're quite close," he says, before acknowledging that it's "been a weird few years" for his family. "It's been quite life-changing for everyone really. It's been quite an adjustment." While his father runs his own business dealing in Formula 1 memorabilia, even turning tyres into coffee tables, Grint tells me the whole clan have "been all over the world" with him for the premieres and promotional duties. "There are some good perks," he grins.

Yet it's clear he's not going to spend much time pining for Potter. Already making further provisions for removing the spell its cast over his life, due later in the year is Wild Target, a remake of the 1993 French film Cible Emouvante about an ageing assassin (Bill Nighy) suffering a midlife crisis. He plays Nighy's apprentice. "The character is closer to me than the one I play in Cherrybomb," he says. "I can probably relate more. He's quite laid back." So it's true then? "I suppose, yeah. I am quite relaxed. Not much fazes me. I don't get angry a lot."

Still, if Grint is looking for a role to eclipse Ron Weasley, he may already have found it. He's currently attached to a project to play Eddie "The Eagle" Edwards, the British ski-jumping record holder who became a hero of sorts for finishing last in the 1988 Winter Olympics. "Nothing's final yet but I'm quite up for it. It's always been quite a big story in my family. My dad's always told me about the legend of Eddie the Eagle. He was a bit of a joke really. But he did actually jump, and set the British record." For the record, he's never skied in his life. "That might be a good thing!" he winks.

A comic tale of a plucky underdog, it rather sums up Grint's career to date. Maybe he'll never stray too far from this comfort zone. But with Ron on the run with Harry and Hermione in the final Harry Potter instalment, at least we'll get to see Grint in a more action-oriented role in Deathly Hallows. "I have hair extensions for the latter part of the film," he explains. "Ron gets a bushier hairstyle because they're living rough and camping out. Me and Dan have got stubble as well!" Ron Weasley with facial hair? Perhaps it shows Rupert Grint is ready to leave Harry Potter after all.

'Cherrybomb' opens on 23 April. 'Wild Target' and part one of 'Harry Potter and the Deathly Hallows' will be released later in the year