An old article from IGN featured Dan talking about Mitchell. You can read below what he said about him.

IGN: An interesting element of the film is that the backstory, and seeing how this relationship began, is portrayed by younger actors. How much did you speak or interact with the actor playing the younger Ig?

Radcliffe: Quite a bit. The stuff was all shot pretty much in isolation, so if the kids were on set then obviously we weren’t there because we never overlapped. But it was wonderful and Mitchell was fantastic and is great in the movie and I know he worked on a lot of things since; he’s doing really well. I always get a kick out of getting to work with young actors because I had a fantastic experience working on film at that age and so I want to try and impart some of that experience with them and hope that they have a good time. Because I know that the experience I had was made by all the people around me so I always try to hope they’re having a good time as well.

IGN: I’m curious about that, because we all know your great stories of working on films with experienced actors who gave you advice. Is it interesting for you now to be in the position where you’re working with this kid and he might have questions for you?

Radcliffe: Yeah, it’s really lovely. It’s really nice. I hope that I can always be helpful and they’re really good. It’s great. Mitch was very sweet as well. Mitch is a little bit more naturally blonde than I am and they dyed his hair, obviously, browner. His first day he had his hair dyed brown, he went back to the hotel. Nobody in the hotel knew that he was doing the film and one of the girls in reception just said, “Oh, you look like Harry Potter!” He was so thrilled. He was so excited. Because it’s a very tough casting challenge to cast somebody to play me at 12 because everyone knows exactly what I look like at 12. So it’s really hard. I think they did a great job.

Source: IGN