I'm not much of a Method actress, so even though my character in The Ghost Writer was quite dark and bad-tempered, I could only do that if I was seeming quite perky.

Joss Whedon writes beautiful drama. His sensitivity and his sense of drama and scenes are pretty exceptional. There's no one else writing like him, really, in sci-fi and TV. That's not to say there are no astonishing writers on TV. I was nervous about coming to America and playing an English person who speaks very English when all the writers are American, because it's a very particular thing to imitate, and if it's badly imitated, it sounds painfully contorted and silly. And he writes very well for English people. It was Joss Whedon who persuaded me.

I'm not much of a Method actress, so even though my character in The Ghost Writer was quite dark and bad-tempered, I could only do that if I was seeming quite perky.

Carey Mulligan is completely self-possessed. She knows what she's doing, and again, quite like the big-hitters I was talking about, my job was just to feel and reflect back the amazing performance coming my way. If anything, I was slightly pathetically flying in her tailwind. She's an extraordinary actress.

It was fine when I was single and childless. Carrying the responsibility of screwing up your kids at the same time is huge. I remember when I got Peter Pan, and I told my mom and dad and my friends I was leaving - again, I was cast way late on - in the next two days to go to Australia for four months, and they all went "Bye! See you in four months!" But no one said "We need you," and I really knew that it was time to think about someone else for a change.

Roman Polanski actually said as much to me once. He had his head in his hands, and I said, "Roman, I've got to tell you, as an actor, seeing the director with his head in his hands... Look, I really want to do what you want me to do." And he went away and he came back, having obviously thought about what I said. And he said, "When my head is in my hands, I'm closing my eyes and trying to remember what I saw in my head, before any of the stuff."

My parents are both lawyers, and my father always said his best cases have been returns, cases that come into his office from another lawyer. So he said, "Never be ashamed to take a return." All my best roles have happened because someone else dropped out at the last minute, and God bless those actresses for their queeny fits, their sprained ankles, their better job turning up, because that's how I've got my best work.