Matt Smith Interview with Vulture

Matt Smith recently did an interview with vulture.com. He confirmed that he'd known when he'd be leaving for a while.

To be honest with you, it was something I was considering for a while. I talked to Steven [Moffat, series boss] about it, I talked to my family about it. I don’t know really. It was a very hard decision to come to because it’s such a wonderful show and I love making it so much.

He also confirmed that the Christmas Special would involve Trenzalore - and River Song.

Eventually Steve peppers it back — you think now we’re going toward the Doctor’s final hour, which is toward Trenzalore. Steven mentioned that in season five. Season five! The Silence was mentioned! He peppers this brilliant storytelling all the way through, and for my money that’s a much more interesting way to write. Aren’t all the layers what we fans of sci-fi want? I’m like, Cool, throw it at me. Is it complicated? Yes. Are we going back in time? Yes. Are we going forward in time? Yes. Are we meeting a character we’ve met in the future? Yes. Is she my wife? Yes.