James McAvoy recently spoke with Comingsoon.net about his work in Atonement:

CS: I wanted to start by asking you about working with Saoirse Ronan. I've seen her in another movie and she's a bit of a scene-stealer, isn't she?
McAvoy: Yeah, she's great, she's really really good. She's a nice girl as well, and she's a proper actress. She's not just like a little girl who's very natural in front of a camera, because that doesn't necessarily mean that they'll be able to do what the script says.

CS: Romola Garai mentioned that when she worked with Saoirse, she would tell her why she did things in a certain way, and I understand her father's an actor as well.
McAvoy: Yeah, he's good too, but yeah, she's a proper actress.

CS: Is it hard to work with someone that young in terms of expectations of having to carry them before finding out they're that good already?
McAvoy: No, not at all. You don't really have expectations like that I don't think, and also because you do three weeks of rehearsals and you just go, "Yeah, she's really good. She can do her thing." Nah, she's wonderful. It's a pleasure working with her and why wouldn't you enjoy working with a good actress, whatever age she is? 'Cause it only makes you better I think.

Click here to read the full interview.