He was the boy who brought Harry Potter to life – and feared he'd killed his own career in the process. Daniel Radcliffe talks about hairy legs, growing up in the spotlight, and why he never watches the films that made him famous

Daniel Radcliffe says he has just been called a national treasure, and it has left him discombobulated. He has a point: most people his age have barely started out. But here he is, at 24, more than half his life spent in the business, 16 movies behind him, eight of them Harry Potter blockbusters. It feels as if he's been with us for ever.

The funny thing is, apart from the fags and the facial hair, he doesn't really look any different from the speccy schoolboy wizard who made his screen debut in 2001. Yet over the past half-dozen years, he has done everything he possibly could to distinguish himself from Harry: riding a horse naked and aroused on stage in Peter Shaffer's Equus, limping around stage as Billy Claven in The Cripple Of Inishmaan, haunted by ghosts in the horror movie The Woman In Black. Now he's at it again, with another part from which Harry Potter would run a mile: in Kill Your Darlings, he plays gay beat poet Allen Ginsberg, sexually infatuated with the dangerous Lucien Carr.

Radcliffe says he was doing a Q&A recently and somebody in the audience asked why he played such weird parts. He looks pleased when he tells the story. "I can't put it down to anything more than that I've got weird taste. There have been plenty of weird roles so far, and more to come, I hope. I pick films based on scripts and directors and parts. I think I've got good taste, but it's slightly left of centre. I'm not interested in making films I've seen before. There's nothing more exciting to me when I read a script than originality. That's all it's governed by; there's no master plan to distance myself from Potter with every role."

But it's telling that he uses just the surname, which has exactly that distancing effect. Oi, show some respect, I say – it's Harry Potter. He grins. Has an actor ever had such an ambivalent relationship with the character who made him famous? For many people Radcliffe is Potter, and Potter is Radcliffe. Potter made him unbelievably wealthy, and he also made his every social outing meat and drink for the tabloids.

We first meet at the West End offices of his agent, just before an evening performance of The Cripple Of Inishmaan. He has a hacking cough that can't be helped by his frequent trips to the window for "a bit of fresh air", or what the rest of us call a cig. He's wearing tight jeans, no glasses, and is a super ball of energy. He is fiendishly polite, slight, well turned out. If you'd never seen him before, you might assume he was a children's television presenter.

Radcliffe says he doesn't want to sound ungrateful. "I know that Potter is going to be with me for the rest of my life, so to try to set a goal where nobody talks about that any more is stupid. It would be like… Paul McCartney might have gone on to do a lot of other things, but people are always going to want to talk about the Beatles. It's just a fact of your life, so you can't get annoyed by it or resent it. You have to embrace the fact that you were involved in this incredibly cool thing that did wonders for the British film industry and though you might not always be happy with the work you did on it, the opportunity it has given you to forge a career for yourself is amazing."

Radcliffe takes his acting seriously. It's ironic that, having grown up as Potter, a leading man being paid leading man's wages (and then some), he is now establishing himself as a fine character actor, the type who plays the second or third lead, or the quirky cameo, not quite handsome enough to be a romantic lead. In short, the type who can make a decent living but would never command the salary Radcliffe did as a teenager.

The two performances I watch in quick succession could not be more different. In Kill Your Darlings he is the nerdy, needy young Ginsberg, desperate to hold on to his gorgeous, enigmatic boyfriend. As Cripple Billy, he is rural Irish, physically twisted, carrying a useless leg and hopelessly in love with a local girl who mocks him.

For once, he even looks different in Kill Your Darlings – curly hair, lips made up to appear fuller, very Jewish. You make a very convincing Jew, I say. He laughs. "Well, I am Jewish. My mum's Jewish. I'm Jewish by blood, but an atheist. There was a great Peter Ustinov quote where he said the Jews have had the luxury of giving the world two of the most influential people, in Jesus Christ and Karl Marx, and following neither of them." Radcliffe is full of quotes. He didn't go to university, but says he discovered the joy of learning for learning's sake when he was tutored on the Harry Potter sets.

Radcliffe in Harry Porter And The Deathly Hallows: 'There aren't many great parts out there for teenage boys, certainly not as good as Harry Potter.' Photograph: AP

As an only child, he spent most of his time talking to adults and has always felt older than his years. His father Alan was a literary agent, his mother Marcia, who was born in South Africa, a casting agent. Both acted when they were children, too, and are now his managers. Radcliffe had already made his screen debut by the time he was cast as Harry, in the movie The Tailor Of Panama, and on television as young David in the BBC adaptation of David Copperfield. He says he was unconfident and largely unhappy at school. He went to posh schools, where you were considered a loser if you were no good at sport. He still remembers a teacher at Sussex House, the independent day school for boys that proudly cites him as a star alumnus, telling him he was stupid when he was eight years old, and how devastating he found that. "At the time I just believed him." His voice rises to a hurt squeak. "I thought, I probably am stupid, but when you grow up, you think that's outrageous for a teacher to say that to a small child… I was probably just talking to somebody or asking for something. I was a very disorganised, talkative boy. And there are some people who just don't do well at school. I am not somebody who will learn best when you tell me to sit down and be quiet and sit still. I learn by talking back and engaging in conversation and walking around."

Is he hyper? "Yes, I think I am quite hyper. I think if I'd been born a few years later, I might have been diagnosed with ADHD, but I missed the boat for all those diagnoses." If you shut your eyes, you could be listening to a sweet old man looking back on a distant past. "I always feel sorry for people when they say school's the best days of your life. It really isn't. If it was, you must have a terrible life." He coughs like a consumptive. "I never understand that. That was one of the things I loved about Potter initially, it got me out of school."

Was he aware how much Harry would change his life when he was offered the part? "No, I knew I was signing on for the first two, that four books had come out. Warner genuinely didn't know at that stage if they were going to make more than one film. If it flopped, then they certainly weren't going to put up all that money again." Did he ever consider exercising his opt-out clause? "By the third film, I thought, if there's a time to get out, it's now; there's still enough time for another actor to come in and establish himself. For a while, I thought, if I do all of them, will I be able to move on to other stuff or should I start doing other stuff now? But in the end I decided I was having way too much fun. And actually there aren't many great parts out there for teenage boys, certainly not as good as Harry Potter."

By the time he'd made the fourth film, and was past the halfway mark, he decided it would be foolish to stop. By then he felt he was mastering his craft. Actually, he says, it wasn't growing up in public that he found so difficult as learning how to act in public. What does he look back at with embarrassment? "God! Loads of stuff. My acting lessons are there for all to see. That's essentially what the films are, us learning to act."

Of the three leads – himself, Emma Watson and Rupert Grint – who does he think was the best actor? "I think we were all good at different things. Rupert was the most outgoing of the three of us; he had this confidence when it came to comedy, and he still does. My tendency is to underplay stuff and sometimes I look at the films and I know I thought I was being subtle at the time, but actually I'm just doing bugger all. That's a learning curve. You can't dwell on the things you're not happy with, though; you've got to move forward and get better."

He's right. At times Harry was so understated, he verged on the torpid. In his most recent work, minimal gestures are used for maximum effect. He nods. "I still don't want to be a big over-the-top actor. That's never going to be the way I work. But I think I've got better at striking a balance where I'm being real but still being expressive."

He tells a funny story about Harry Potter and puberty. "In the second film, there was a shot where I had to hitch my trouser legs up to show I'm not wearing socks, and at 12 my legs were way too hairy, and they said, 'Nobody's going to believe any 12-year-old would have that much hair', so they had to shave two inches up from my ankle."

Radcliffe might play down the impact of growing up in front of the cameras (both movie and paparazzi), but talk to him long enough and you realise it has had a considerable impact on him. After all, armies of reporters were assigned to chronicle every drink, scrap, falling out, romance, rumour in the lives of Radcliffe, Watson and Grint. There were stories of him being drunk, stoned, depressed, going off the rails. In the past, he has said that at 18 he lost it, was drunk on set and blacked out through alcohol. He stopped drinking in 2010. Today you couldn't imagine a more functional, scrubbed-up 24-year-old.

I tell him I've read loads of pieces where interviewers ask him about drink and he goes all surly. "It's because I won't talk about it any more." Were the stories exaggerated in the first place? "No. Nobody but me knows what happened. The time I talked about all that initially was a huge mistake. I thought the narrower the gap between people's perception of me and the reality of my life, the better, but actually, the more information you put out there, the more people have to speculate on. And I don't like being talked about, I don't put myself out there to be judged. I made the mistake of doing that the first time."

There are enough stories about Radcliffe without adding to them himself, he says. Just recently, there was a picture in the Daily Mail of him leaving the theatre looking dishevelled, accompanied by video footage of him "playing the bongos during a wild night out". He laughs. "I was walking from the theatre to my car, and it was 'Daniel Radcliffe looks exhausted and frustrated' and it's like, yeah, cos I had your ugly pap in my face. There's stuff like that that gets annoying even now, but it's more than worth it because I have a great job, and dealing with the tabloids actually comprises a very small part of my life."

While making The Woman In Black, Radcliffe developed the Fassbender test: 'If you're ever being asked to do anything, you just ask yourself: Would Michael Fassbender do it?'

Was there a point when he thought, get me out of here, I've had enough? "Not of filming. There are certainly times when the fame aspect gets on top of me, and gets overwhelming." It was at its worst when he was 17 and 18, and his friends were beginning to enjoy life as young adults. He says he didn't have the same freedom to experiment and make mistakes. "I was very frustrated about the fact that I couldn't just go out with my friends and have an easy time." What would happen if he did go out? He looks embarrassed. "Well, you know, I'd get mobbed, then people start taking photos, then they go on the internet, then everyone knows about it, then more people arrive because they've seen it on the internet."

Did he get verbal abuse? "Yeah, absolutely. And there are a few guys who come up to me and they've taken their brave pills, so they think they're going to be really witty and tell a Harry Potter joke, and I just stare at them now, like, are you really doing this? I've heard every single one, you will not make me laugh." He tells me a typical joke. He's outside having a fag, it's pouring down, and some wag will ask if he's got a spell to banish the rain. "People always look at me as if I'm expected to laugh at their brilliant joke." Then occasionally strangers would want to beat him up, just for being Harry Potter. "You'd get a few people, or drunks, who'd want to pick a fight with me, but again I didn't want to give them the satisfaction. On the whole I'm very lucky because I'm part of a franchise which is really well liked. I'm not a fighter. I believe I'm articulate enough to talk myself out of most things."

Part of the problem, he says, was that he felt so self-conscious: even if people weren't staring at him, he felt they were. "It was at the stage where all my friends were going to clubs and under-18 nights. I couldn't because there's a level of anxiety I get when I go out. Even if the people aren't remotely interested in you, it's in your head, and if you start dancing, you think everyone's going to say, look at Harry Potter, dancing like a twat."

Was he scrutinised more than Watson or Grint because he was Harry? "Not at all. Because of Rupert's red hair, it's hard for him to go out without being recognised. I watched him when we were at Reading festival one year and he walked across the VIP area and everybody turned around because he sticks out. And Emma gets attention because of the rabid male fan-base she has."

Last year Watson burst into tears during an interview when the journalist quoted bits of an unauthorised biography at her. "The book is total fiction," she said. Radcliffe says he didn't see the interview, but he knows how she felt. "There was an unofficial biography of JK Rowling, and the writer referred to my dad, who was an actor when he was young, as an 'unremarkable actor' and I'm like, you've never seen him act, dickhead, you're basing that on nothing."

And, of course, there is the perennial fascination with the private lives of the Potter three. Radcliffe says he doesn't discuss relationships with the media. There have been rumours about his sexuality, not least because of the roles he has taken on. I ask him if he's gay or straight. "I'm straight, yeah. But I've been called gay since I started working with the Trevor Project [an American suicide helpline for gay, lesbian, bisexual and transgender youth]. Everybody just came out and said, oh well, he's gay. The best comment I read about myself online was 'He's got a gay face' which was all the confirmation they needed."

He couldn't care less what people think of his sexuality, and as for intrusions of privacy, Radcliffe is the first to admit he's got off relatively lightly. Not only does he have a career he adores, he is worth an estimated £60m, according to the Sunday Times Rich List (more than double either Watson or Grint), making him the 10th wealthiest person under 30 in the UK.

The pluses so obviously outweigh the minuses, it's not even worth asking whether he'd do it all again, given the chance. There is something I'm curious about, though: does he ever watch himself in the Harry Potter movies? He looks appalled. "No! Never!" he splutters.

The reason I ask, I say, is it must be confusing at times: if, say, you watched a movie you made at 16, would you say there's me at 16 or Harry at 16?

"I'd look at it and say there's me at 16," he says instantly. "I know I would because my memory of those 10 years is very vivid, and I remember things from every day that we shot."

A few weeks later, we meet at a photographic studio in east London: the sun is shining, Radcliffe's cough has gone, and he looks more sprightly than ever. We try to dress him against type for the shoot – unshaven, vest, smoking. And the harder we try, the more difficult we realise it is to escape Harry Potter. Occasionally, Radcliffe does something to remind you how young he is: while everybody else eats chicken and rice, a massive pepperoni pizza is ordered in for him. He has brought in his own playlist for the shoot (Black Keys, Jon Spencer Blues Explosion, M Ward, Pixies, Hold Steady, Elliott Smith, Ball Park Music) and is chatty and relaxed. He tells me of a test he has devised to assess whether a role is worth taking. "I developed it with James Watkins who directed The Woman In Black, and we call it the Fassbender test. If you're ever being asked to do anything, you just ask yourself a question: 'Would Michael Fassbender do it?' This referred to a moment when I was being asked to do something a bit more corporate than I'd ever done before, and it came down to the Fassbender test, and we thought he wouldn't do it. It's a pretty good test for a lot of young actors."

Radcliffe as gay beat poet Allen Ginsberg and Dane DeHaan as Lucien Carr in Kill Your Darlings

One thing he prefers about making smaller films these days is that it's over and done with so much quicker. He talks about the final scene in Harry Potter which took four months to shoot. "It's almost impossible to film a scene for four months. It's a great scene, but it does make it harder to keep energy up for that time. We filmed Kill Your Darlings in 25 days; nine pages a day, which is faster than some TV shows. And that's exciting, you've got no time to slow it down. I loved it. I wish all my films could be done at that pace."

Since last seeing Radcliffe, one thought has dominated: how strange it must be to be so rich at that age. True, he has homes in west London and New York, but he doesn't look like a man who spends money on clothes or cars or anything flash. Most of us wouldn't mind a million or two, but perhaps being so wealthy is a bit of a curse.

Did he realise he was going to be incredibly wealthy if he made the Harry Potter films? As soon as I mention money, he blushes. "No, absolutely not. No, I didn't find out how much money I'd make till I was 19." I tell him I'm trying to find a suitable description for his financial situation, and the only expression I can think of is filthy… He finishes off the sentence for me: "…stinking rich, yes, that's the one. It doesn't matter what you do, no matter what line of work you're in, you can never really deserve that amount of money. It just so happens I'm in an industry where those sums of money are dealt with." He's reddening by the word. "And I'm very lucky to be in a position where I don't have to do a job just for the money."

What's the most extravagant thing you've bought? "Art. I suppose that's an extravagance; you don't need a painting." How much did you spend? "I won't say, but it was a Damien Hirst butterfly painting. But other than that I'm just so dull with my money." He's been sensible with his earnings, he says, invested in property.

I bet when you go out to eat, you get the biggest prawns, I say. He laughs. "Yes, gambas gambas prawns! Actually I only got an HD TV a couple of months ago.

"Money is a wonderful thing because it gives you room to manoeuvre and breathing space to choose whatever you like professionally, but, yeah… I'm very lucky… yeah. It's embarrassing to talk about." Now he really is struggling for words. "It's embarrassing that it would have some impact on the way people see me. That is not something I enjoy." Does he ever wish he had less money? "Only because it would come up less. Most people in life would agree money is a topic that is not appropriate to bring up. You'd never ask anybody else how much they make, but because I am in a position where you are 'filthy rich' from a young age, it becomes a curiosity. I'd hate for someone to make assumptions on me based on what my bank balance is. I'm sure some people do."

I wonder if he has trust issues. Does he ever wonder (to paraphrase the classic Mrs Merton question) why someone might be attracted to the multimillionaire Daniel Radcliffe? I feel cruel even asking the question. He giggles, and says people are going to be sadly disappointed if they befriend him for his lavish spending. "Anyone who is my friend knows that I don't spend money. So they can hang around with me as much as they like and they still ain't going to get anything. Haha! They barely even get a birthday gift – if they're lucky." But, he says, he has never had a problem with working out who to trust. "I'm a fairly good judge of character, and I have a small but very close circle of friends. I'm not looking to recruit new friends. I've got enough, they're fine, I love the people in my life. I'm actually very open with people. I had a similar conversation with myself when I was about 17, the first time somebody had really betrayed that trust, and I said to myself you have two options: you either become totally insular and shut down and not let anybody into your life ever, or you can continue to be open and amiable when you meet people, and trusting, and occasionally get screwed over. And I do think that is the best way."

Anyway, he says, there are amazing things you can do with money. He talks about the charities he works with – as well as the Trevor Project, there's the Demelza hospice and the Get Connected helpline for young people. "So the most valuable thing I do with my money is that. The only thing of real import."

Could he imagine a day arriving when he quits acting to become a full-time do-gooder? Now he looks as shocked as when I suggested he sits at home watching old Harry Potter movies. "No, acting's too much fun. I love it too much to stop. And it's what I do, what I've grown up doing." He's working on a number of movies, including Frankenstein (as the hunchbacked lab assistant Igor) and Gold, a film about the rivalry between Sebastian Coe (whom he plays) and Steve Ovett. "There's an assumption that people make where if you've grown up on a set, surely you must be tired of that or want to go somewhere else. But actually I've never known anything but being on film sets and I love them. They are places of immense comfort and familiarity. I hope I get to die on one. Honestly." He smiles. "Later rather than sooner."

• Kill Your Darlings is released on 6 December.