When you're making a psychological thriller, what you need to do is have an audience on shifting sand so they're never quite sure where they are.

In the film industry you never really know if all the various ingredients will come together - sometimes they do, and sometimes they don't. As an actor, you don't have much control over those things. It's a director's medium in that sense. All you can really do is minimise the risks of being involved in something that might not work and look for something that also suits you.

Rowan Joffe was on top of everything. He knew exactly how he wanted everything to be, right down to the fact when I made those telephone calls, I didn't do it afterwards in a studio. I think that's how he operates. He likes it to be as real and as clear as possible.

I think it serves the purpose of the film if the premise is that you're unsure of me because you've only ever really seen me play villains.

[Before I Go To Sleep] script was a great journey with all the twists and turns that were kind of unexpected. I had to finish the script, and I thought if we can emulate this in the film, it's going to be a really good film.

The idea of transformation - playing something I'm not - is the bit I enjoy most about acting.

I'm not a writer, inherently. Most of the writers I've met have stories they need to tell. I don't have that. I'm an interpreter. I like getting a script, seeing a character and thinking, "Oh, wow, I know what I can do with that."