While promoting Half-Blood Prince James and Oliver Phelps talked about Harry Potter, duh ;).

You were 14 when you were on the audition, what you thought about the books then?
Oliver: I had read all that had come out at the time, Goblet of Fire was the last. It was slightly surreal because when you read, you can see the characters for themselves in the head and I saw James for me and I like Fred and George because we were twins. And when we started filming I was that I began to see me for the cast as Dan, as Emma, as Rupert and all the others, and so did I still when I read Deathly Hallows two years ago. – The difference is only that when you read that they go from the story to living through the corridors, for me as it is to exit a scene, over the parking lot and into another. It is quite strange.
Why exactly were you were chosen?
James: Janet Hudson said she had raised eyebrows as soon as we entered the door. I think we had the look they were after.
Oliver: We had a test in front of the camera not long after with the director Chris Columbus, who was American. He asked who of us who would win a “fight”. “Fight” on the U.S. means a discussion, while in the UK means the fist fight.
James: … So I said “I would win, I have a pretty good left hook”, and he not only understood what I was talking about. There was much that was new for us in the beginning.

Thanks to SS for the quotes.