Your latest role is in Martin McDonagh's play The Cripple of Inishmaan.
What made you want to play Billy, the "cripple" of the title?
Michael Grandage, the director, presented me with three or four plays, and as soon as I read Cripple there was no contest. I'm very much the tragic relief of this play: Billy has a few funny lines but a lot of the comedy comes out of people being incredibly cruel to my character. Which I'm very, very happy with. I've learned that I really enjoy stage violence. I was lucky enough to spend a lot of my lunchtimes as a child choreographing fight scenes on Potter. So I'm quite good at it: the stunt department always said that I bounce.
Billy dreams of escaping Inishmaan for the neighbouring island of Inishmore, where they are making a film – the real-life, 1934 Man of Aran. Did you feel any personal connection to the role?
Billy's ambition to get away from the island is definitely one of the things I find really attractive. Somebody who's been so beaten down all his life can still say: "No, you're all wrong. I believe I can make something of myself." I absolutely don't relate to being beaten down my whole life – I had amazing opportunities at a young age – but there is still in many, many people's minds the notion that I'll never be able to escape Harry Potter. So my drive is to prove to people that I'm not a one-trick pony, basically. Drive is not something I'm lacking in.
Yeah, that's the only thing I knew about it: I'd watched maybe the first 10 minutes along with the album. I mentioned it to Michael in the hope that I might get to meet British Sea Power, bring them in to soundtrack it.
Is it true that you want to have a tattoo of a British Sea Power lyric?
Yes: "Bravery Already Exists". I want to get a tattoo on my forearm, something on my chest and maybe something on my back. There's various quotes I love. It sounds pretentious but there's something about the Beckett quote: "Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better."
But you don't have any tattoos yet?
No, I've got to get a sustained series of jobs where I don't have to get naked. But I haven't hit one of them yet, for some reason. There was that first few years of my career when I was just doing Potter where there wasn't a huge amount of nudity. But since then it's been almost every job. Recently I had my gay sex scene in Kill Your Darlings, a skinny-dipping scene in The F Word and a straight sex scene in Horns. It's just been a year of it.
From Equus onwards, you've picked some intrepid jobs. Do you need to be scared by a new project?
In a way it's just bad luck. Everything that's really interesting to me happens to involve some massive, scary thing, like getting my dick out or learning to dance [How to Succeed in Business Without Really Trying on Broadway]. Perhaps it's having spent 10 years in what could be described as a comfort zone. Not to just throw these slightly cliched phrases at you, but there's a Henry Ford quote: "If you always do what you've always done, you will get what you've always got."
What is it you like about the Beckett quote on failure? The Harry Potter films made billions and both the Woman in Black film and TV drama A Young Doctor's Notebook were hits…
I know it wouldn't seem like I've had a lot of failure in my career but there are things that I regard as failures, when I look at certain performances and go, "That's not good enough."
Are you going to say what those are?
The sixth Harry Potter film – I don't like my performance in that film at all. There's stuff in The Woman in Black: I'm really glad it did well but I look at that film and there were probably six weeks between finishing Potter and starting that, so I hear the same voice and I see a very similar style of acting.
It does seem to amaze everyone: "Why aren't you fucked up?" It's great when you come in at that place of very low expectations because you'll always be a pleasant surprise. But it's so hard to pin down why we all turned out all right. You know, with my mum be a casting director and my dad a literary agent, I've heard a lot of round-the-dinner-table complaining about certain actors and their behaviour. So when you come to a film set with that in your head, you go, "Oh God, I don't want anyone to ever talk about me that way."
You once said you watched a lot of parliament on TV. Do you still?
Parliament less so nowadays but I watch tons of quiz shows and I'm a little bit of a Come Dine With Me obsessive. I'm always amazed because I'm a very unadventurous eater but why would you go on that show and seemingly eat nothing? They just go round to people's houses for a week going, "Ooh, I can't eat that."
In what ways are you an unadventurous eater?
This is one of the things I could never admit to while I was on Harry Potter but I'm very bad at fruit and vegetables. I basically have the diet of a 19th-century Irish navvy, apart from the litre of stout a day. It's meat and potatoes and bread and cheese: those are my four food groups. But at the moment I'm doing enough exercise, so it's fine.
In your next film to be released, Kill Your Darlings, you're playing Allen Ginsberg. Where was that on the scale of terrifying?
That was pretty fucking terrifying, I've got to say.
Because of the gay sex scenes?
No, no, no, just because of playing Ginsberg in a film about the Beats. Allen Ginsberg: American, Jewish, working class. Daniel Radcliffe: English, Jewish by the way but not really, upper class, and definitely looks a million miles away from Allen Ginsberg. When you start from that you think: "I can't just be good in this film. I have to be better than good for people to even actually give me the chance." But I think a lot of people are going to see it and see me in a very different way afterwards. Which is a good thing.
The Cripple of Inishmaan runs at the Noël Coward theatre, London from 8 Jun to 31 Aug
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