How will Daniel Radcliffe's fans react to his latest turn - as a damaged, drug-taking Beatnik poet?

fter a decade of playing JK Rowling's boy wizard, it is perhaps understandable when Daniel Radcliffe announces that Halloween has become his favourite day of the year. As one of the most recognisable young men on the planet thanks to his starring role in all eight Harry Potter films, the chance to put on a mask and walk around in perfect anonymity is like a gift from the heavens. "It's the most surreal feeling, to be able to walk with my head up and look in people's eyes," he says. "It's very bizarre. "The fun thing to watch is me walking through a crowded street, particularly if I'm with my girlfriend, because it looks as though she's my carer. I basically keep my head to the ground and follow her feet."

Even though he is 5ft 5in and thus, one might imagine, easy to miss, Radcliffe still gets spotted. Like the time he stood in the wings at a Red Hot Chili Peppers concert in London's Hyde Park, only for word to spread among the crowd that he was there. Soon enough, there came the chants: "There's only one Harry Potter."

Yet, however bizarre he professes his life to be, Radcliffe, who celebrates his 23rd birthday next month, also seems entirely at ease with his fame – which is fortunate, because it isn't about to diminish just because there are no more Potters in the works. His first post-Harry film, The Woman in Black, recently became the highest-grossing British horror in 20 years, taking more than $127m around the world.

I've met Radcliffe several times over the years, but I'm always left with the same impression: frightfully well mannered, genuine and good-humoured. "He's the sweetest, nicest multimillionaire I've ever met," laughs the actor Peter Mullan, who worked with him on the final two Potters. Even invasions of his privacy rarely ruffle him. "The only time it pisses me off is when people take pictures without asking," says Radcliffe. He recalls an incident in a New York restaurant: "I was there with my girlfriend, and this girl didn't even look, she was carrying on talking to her friend, and she had a cameraphone [casually pointing away from her] and this huge flash from two tables away went off. I was in such disbelief. So I waited until the end of the meal, getting angrier and angrier. At the end I went up to them and said, 'If you had asked, it would've been fine. What you did was rude – just for future reference!'"

The girlfriend, whom he can't help dropping into conversation, is Rosie Coker; she's a production assistant, and the pair met on the Potter films. She's the same age as Radcliffe, and "takes it all very well", he says, meaning the attention that's been thrust on her. He told the American magazine Parade that he was "crap" at dating – and, having lost his nerve when trying to kiss Coker on their second date, he wound up accidentally smooching her neck.

His personal fortune may be estimated at close to £50m, but it's stories such as this that make him sound just as gawky as any lad his age. What's more, his sex-symbol status is baffling to him. "It's fantastically flattering. I don't see it myself, but great," he laughs. "If that's how girls want to see me, then I'm delighted with that, and I certainly won't be complaining or trying to dissuade their illusions."

Dressed today in a chunky grey cardigan, white T-shirt and green combat trousers, with the slightest hint of stubble etched across his face, Radcliffe is looking well – though endless promotional duties have taken their toll. One of our interviews was cancelled due to ill health. "I was in the middle of these weird things called 'cluster headaches'," he explains. "It's very rare evidently, but it's exceptionally painful. It makes a migraine look quite tame. At the time I thought I was being a wuss. I was taking 12 [strong painkillers] a day and going, 'Why do I still have a headache?'"

In the end, a trip to the doctor and a series of injections eased his pain. Hopefully, this revelation won't generate the same furore that came when he announced that he had stopped drinking in August 2010, after the booze had started to become an issue. Much of this coverage came during the making of Harry Potter and the Half-Blood Prince, when Radcliffe had turned 18. Tales of whisky benders emerged, and the actor confessed he drank nightly. "I never drank at work," he said at the time. "[But] I can point to many scenes [in Half-Blood Prince] when I'm just gone. Dead behind the eyes."

At the time, the papers ran with "Harry Blotto" headlines while his Potter colleagues rallied round to show solidarity. Astonishingly, David Yates, his director on the final four Potter films, told me: "He's a teenager. Most teenagers in the UK have a drinking problem. In fact, most adults in the UK have a drinking problem. He's just like any one of us in that sense. He kept it pretty well managed as far as the work is concerned. It's a very normal part of growing up and he dealt with it, at the same time as being under this extraordinary pressure."

Perhaps it was more of that good fortune, but Radcliffe avoided tabloid scandal through his boozy period. Now, entirely loved up with Coker, his head is in a good place. It helps that The Woman in Black was a hit. Was he surprised? "I wasn't," he claims. "I always knew if it was done right it could be a film that a lot of people will enjoy and go and see."

Admittedly, his popularity and the enduring appeal of Susan Hill's chilling Victorian ghost story – already a hugely successful play – meant it always stood a chance. Showing Radcliffe in a more mature light than playing Hogwarts' Harry, the film cast him as a father and a widower. "Two things I don't have direct experience of," he admits, shrugging, "although I absolutely love kids. I think they're amazing and I can't wait to have some – but I will wait!"

If it looks calculated, he claims he hasn't spent time plotting his exit strategy from the world of Potter or worrying about being typecast. "That's not going to be very constructive for me," he says. "It would be a waste of energy."

But he's not so blinkered that he is unaware of the intense interest surrounding his every decision. "I get it," he says, "but I don't think I'll ever let that inform my choices. As much as anything else, it's about showing people you've got good taste. If a Brad Pitt or George Clooney film comes out, I'll go and see it because I know they've got good taste. So I want to be one of those actors where people go, 'I don't know what this film's about but he's in it and he normally picks good things, so it must be good.'"

Even before The Woman in Black, Radcliffe had proved that good taste – from baring everything on stage as the disturbed boy Alan Strang in a celebrated production of Equus to singing and dancing in the Tony-nominated Broadway revival of the musical How to Succeed in Business Without Really Trying. There's been a healthy dose of self-mockery, too, playing a lusty version of himself in Ricky Gervais's Extras and an equally unhinged appearance in an "interview" written, directed and produced by Judd Apatow on the website Funny or Die.

He's now just back from New York, after completing the indie film Kill Your Darlings. Set in 1944, it gathers together several key members of the Beat Generation – Allen Ginsberg, Jack Kerouac and William Burroughs – long before they would become literary icons. "I think what will be really exciting for people is seeing these three guys before they were the venerable old men of American literature," he says. "They were running around New York, tearing it up, doing drugs, having this wild life and becoming liberated."

Cast as Ginsberg, Radcliffe is arguably a part of one of the hippest casts assembled this year. Ben Foster (X-Men) plays Burroughs, Jack Huston (Boardwalk Empire, grandson of John, nephew of Angelica) is Kerouac, Elizabeth Olsen (Martha Marcy May Marlene) is Kerouac's first wife Edie Parker and rising star Dane DeHaan (In Treatment, True Blood) is Lucien Carr. "It's the story of that time in their lives and it's the story of them all meeting. And it's the story of Allen falling in love with Carr, somebody who was both incredibly charismatic and damaged."

Radcliffe playing a gay, drug-taking literary icon – what will the Potter faithful think? "That's the great thing about having the fanbase that Potter does: they're constantly ageing and getting ready for new [things]," he argues. "And also the thing about Potter is that it encourages exploration of literature, so out of all the groups of fans out there, I think the Potter fans are going to be the most willing to see us in other things and support us, because they really feel like they've grown up with us."

He's cautiously excited about Kill Your Darlings; you sense that shooting in the US and playing a libertine let him off the artistic leash. "If it ends up being as good as it felt at the time, it could be a really good movie. It was an experience as an actor I'd never had before. I really felt like I worked in a way I had never done before. I think, for me, I've done the best work of my career on this film."

Radcliffe's own background has a bookish flavour. His father Alan is a former literary agent, who gave up his job to chaperone his son when he was chosen from thousands to play Harry Potter. By this point, he'd already acted in a BBC adaptation of Dickens' David Copperfield, after his mother, Marcie, a casting agent, put him forward for an audition to improve his confidence. "I wasn't particularly good at school," he says, recounting his time at the Chelsea-based Sussex House. "I found it very hard to concentrate."

Yet, taught on the sets of Potter, Radcliffe developed a thirst for reading that would put most undergraduates to shame. "I do owe Jo Rowling a certain amount of credit, in that the Harry Potter books were as responsible for getting me into reading as they are for a lot of kids." But he also owes the Divine Comedy's song "The Booklovers", with its endless namechecks of literary icons. "I took it upon myself to try to read at least one book by every author on that list. I have done about 20 of them."

Not mentioned in the song is Mikhail Bulgakov, the Russian author of The Master and Margarita, Radcliffe's favourite book. So obsessed is he, that on a recent press tour to Russia he conducted an interview at Bulgakov's flat. Now he's set to participate in A Young Doctor's Notebook, a television adaptation of several Bulgakov short stories. Commissioned by Sky Arts, the mini-series sees Radcliffe join forces with Mad Men star Jon Hamm, who will play the same character in an older guise. "I'm immensely flattered by the thought I might one day turn into Jon Hamm!" he smiles.

Unlike his Potter co-star Emma Watson, Radcliffe is not planning a late detour to university. He needs that womb-like safety of a film set. "I've been on film sets since I was 10. I can't imagine what my life is without them. All the growing-up moments happened to me anyway, just on set. It was just a different way of growing up. So I don't think I necessarily missed out on anything." He points out that David Yates says he's been conditioned to work. "I don't mind that. If that's the flaw I have, that I'm impossible to stop working, that's fine with me."

'The Woman in Black' (12) is available on DVD from tomorrow. 'Kill Your Darlings' will be released next year