I got to read some writings by serial killers, and they got inside my head. They were quite disturbing. I read disturbing stuff about that very detached way of manipulating people to do things.

People fear they won’t get what they want.

I have a great editor and I enjoy, in a masochistic way, being ruthless about my own performance. How do I know, but I think I'm quite good at saying, "That's no good. That's no good. That's it. That's it. That's good." And I'm with the editor who goes, "No, I think you're wrong. That's not your best." There's an initial point in the editing, if you're directing yourself, especially in my case, where you go, "Ouch, ouch, ouch, I can't watch this." And then, there's a point where you become hard-nosed and just take your neurosis away and go, "What's working? That's okay. That's okay. We can lose that, and lose that." You get objective about it.

You have to just dive over the edge. You haven't got time to mess about.

I should say, a piece of advice that was given to me very early on by the principle of RADA (Royal Academy of Dramatic Art) which is where I went. When he auditioned me, he said, "Your speech, monologue, is fine. It's good. Yeah, I think you have ability but you're making it happen. Don't make it happen, let it happen." And that's a sort of subtle shift I think, as an actor.

He's really sort of the devil. He's completely emotionally detached. He has no empathy. You find that in psychopaths. It's about power with Voldemort. It's an aphrodisiac for him. Power makes him feel alive.

I loved doing Harry Potter. I had a great time on that. And I think it's good to have a mix. I think you're learning all the time.