He explained that he is taking a back seat in the search for the Time Lord's new outfit, as he failed to spot the potential of Matt Smith's bow tie.
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"[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," the Radio Times quoted Moffat as saying.
"I'll just let them get on with it and it'll be 'yes' or 'no' at the end," he added.
The head writer admitted that he objected to the idea of The 11th Doctor's bow tie as he thought it was a "cartoon idea of what Doctor Who is".
"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."
Earlier this month, Capaldi shot his first scenes as the Doctor as Matt Smith completed filming on the upcoming Christmas special.
Doctor Who will return to screens on November 23 for the 50th anniversary special. 'The Day of the Doctor' stars Smith, David Tennant, Jenna Coleman, Billie Piper and John Hurt.