Hitchcock is a big ask. I am playing someone significantly older than me and someone significantly bigger than me. The stuff I find very interesting is why certain physical things have come about. How can he be light on his feet when he is so big? How can his weight vary so much? Where does this rather beautiful voice come from?

The thing about Hitchcock which is quite extraordinary for a director of that time, he had a very strong sense of his own image and publicizing himself. Just a very strong sense of himself as the character of Hitchcock.

I do always feel very proud and flattered by being asked to be a part of American productions playing American characters.

What happens is some of the characters I've played have voices that are so different from my own, that it would be ridiculous if I would dip in and out of that voice in between takes to ask for a coffee, or something. Or to gossip about whatever was going on.

But I think the children of actors share a certain pragmatic approach. One is denied some of that 'running away with the circus' element of being an actor.

Often jobs are un-turndownable even before you read the script. You go, "Well, I have to do that."

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.