And the head writer revealed that he was keeping his nose out, having previously got it wrong regarding Matt Smith's bow tie.
"[Peter] was texting me 'I'm out with [the wardrobe stylist] at the moment, it's going quite well' and some descriptions of clothes I did not understand if it's not a suit," he told the Radio Times.
"I'll just let them get on with it and it'll be 'yes' or 'no' at the end.
"Matt said 'I should be a boffin, I think I need to go with the bow tie' and I said 'No, absolutely not, you're not wearing a bow tie - that's a cartoon idea of what Doctor Who is… Oh, you are going to wear a bow tie - you look incredible in it.
"And from that moment on he suddenly came to life, and he put the tweed jacket on and suddenly he's leaping round the room with a biro pen and that was it - he was The Doctor."
Meanwhile, in other Doctor Who news, it has been reported that Ben Wheatley will direct the first two episodes starring Capaldi as the 12th Doctor.
Kill List filmmaker Wheatley told Screen Daily: "I am very excited and honoured to be asked to direct the first two episodes of the new series of Doctor Who. I've been a fan since childhood.
"I'm really looking forward to working with Peter Capaldi and finding out where Steven Moffat is planning to take the new Doctor."
Matt Smith will be bowing out after this year's Christmas special.