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.

The film depends on the audience's belief in this relationship.

Performance is made in the editing room, and I've come to see the truth in that - the idea that they say performances are usually made in the editing room because what you film is the raw material. I think just going through the process of saying, "Which take do we use? Why is that the take we want? I want that take can you edit again, I'm not sure that's the one, I think it's this one." And just because you go through that process, I think somehow it's made me sort of more open about the [actor's] possibilities.

The actor shouldn't edit themselves or be anxious. And the actors that I admire are always the ones who are inventive and their imaginative life in free-willing. It's a director's job to go, "No here, don't do that, go there."

I think Shakespeare is like a dialect. If I heard a broad Scots accent, I'd probably struggle at first but then I'd start to look for words I recognise and I'd get the gist. I think Shakespeare is like that.

Like anything to do with trust, in any relationship it emerges and just the chemistry, vibe between two people an actor and a director. You just know whether this is someone I like to be with whose interaction with me I believe in.

Voldemort? That might be real.