Steven Moffat has said that as soon as people imagine Peter Capaldi as Doctor Who, they can't see anyone else in the role.
Steven told Broadcast magazine how he knew that he had chosen the perfect successor.
He said: "We couldn't have had another quirky young man because Matt had nailed the part. Peter's already a bit of a national treasure, an incredibly skilled, fine actor.
"As most people have realised, the moment you think of him in the part, you find it hard to think of anyone else."
Steven added that Peter had always been a great supporter of the show and also of Matt Smith as Doctor number 11.
He said: "Every time I was at a TV do and Peter was there, he'd come over to talk about Doctor Who and he was the first to congratulate Matt. I started thinking, 'What if that genius actor might actually say yes?'."
A 50th anniversary special episode of Doctor Who, The Day Of The Doctor, will air on November 23 before Matt Smith bows out in the Christmas episode.