Best ever was filming in Barcelona last year, and I had a couple of scenes with De Niro. He's a very shy man. Speaks so quietly that people tend to bend down and adopt the same tone, almost the same voice, whenever they talk to him - watching, you'd think someone's offering to carry out a hit for him when they're just offering him a cup of coffee.

It's hard to imagine anyone interested in film not being a fan of Alfred Hitchcock because he's such a key influence on the entire history of cinema - it's hard to escape his shadow.

When I told people I was going to be doing the movie and the voice of Dobby, they were kind of awestruck, the people who knew about Harry Potter. I felt rather guilty that I didn't really understand the scale of the job I was about to take on. Now, I am well aware of what I'm doing, and actually, it feels a very serious acting responsibility.

There's something in the rhythm and roll of it that is connected to the way Hitchcock thinks and moves. Then there is everything he ingested - the cigar smoking and drinking that's imprinted on his voice.

The prosthetics were interesting because the artist was so good that they could just put a Hitchcock mask on me, but you don't want to do that. You're an actor playing Hitchcock, so it's about how much of that you're going to do.

When you're training as an actor, a lot of the big work you're learning is to treat fictional characters like real people. You don't have the problem of discovering a backstory with real people, but there's always a mystery which is common to both fictional and factual characters. They are never quite the person you think they are.

I know, for a fact, that I'm a very different person on my own than I am with someone else. We are different with each other. These things are constantly adjusted. And that's true of humans. That's not just true of famous people.