In the wake of Capaldi's unveiling, Digital Spy and other press spoke to Doctor Who showrunner Steven Moffat about how he picked his new "magnificent leading man"...
When did Peter Capaldi first come on the radar for you?
"Quite a while back. I happened to know that he's a very, very big fan and there's something rather seductive about an utterly brilliant, arresting-looking leading man - one of the most talented actors in Britain - who you happen to know is a big fan of the show. You do start to think, 'Maybe we should do something about that!'"
Did have a shortlist of actors you were considering for the 12th Doctor?
"Yes, the list went, 'Peter Capaldi' - it was a very short list! Honestly, there was only one audition this time... and it wasn't an audition so much as... he came round my house and we put him on video to see what he looked like as the Doctor and, gosh, he was terribly good!"
Were you disappointed that rumours of Peter's casting leaked online?
"Does [the secrecy] matter? No, not very much. It matters that people love the choice, and what we were looking at when we saw that there were some hints out there that it was going to be Peter... everyone was rapturous about the idea.
"I said to Peter before he went on tonight, because he was a little bit nervous obviously, 'Imagine if you weren't Peter Capaldi and you had to walk out tonight - you'd be mown down! It has to be you!'"
Is it true that you considered Peter to play the 11th Doctor?
"He did flick through my mind when we were replacing David [Tennant] and it didn't feel right at all - if you think about that, that wouldn't have been right, not then.
"But there's something about Matt's Doctor that paves the way for Peter's Doctor somehow. The important thing to remember about the Doctor is that it is one character, going through his life, played by a succession of different actors. and you have to get to that place each time.
"I can somehow absolutely believe that the strange old/young Matt Smith will turn into the strange young/old Peter Capaldi."
'Doctor Who' - The 12 Doctors:
Was there a particular moment in Peter's audition where you knew you had your 12th Doctor?
"I don't think it was anything particular - you just know when the Doctor is in the room. I mean, I've seen the Doctor not be in the room when people say those lines out loud.
"But you just know - and just to make sure we weren't mad, we showed [the tape] to Ben [Stephenson, BBC Drama Controller] and so on, and we all just thought, 'Well yes, obviously that's it – the Doctor is in the room'.
"When you watched him walk out tonight, didn't you just think, 'Well, there he is! That's the Doctor!' - suddenly he's the Doctor and Malcolm Tucker is knocked out of the way. He's just suddenly this magnificent leading man."
Was it a conscious decision to go for an older Doctor?
"Not particularly. I mean, the apparent age of the Doctor makes no narrative sense at all – he's been anything from his 20s to his 70s. Obviously he doesn't care, he just sort of picks a face off the rack and goes with it!
"I think it's good that we've got a different age, just because I cannot imagine what somebody in their 20s would do with the Doctor after Matt showed us all how to be a 20s Doctor. I don't know what you would do after that, because he was so perfect.
"You'd have to be an alternative or a deliberate contradiction - it wouldn't work, I don't think. So it makes life easier, I suppose, that Peter is different. But that wasn't the reason, the reason was... the Doctor was in the room and that's it. You don't argue with that!"
What sort of Doctor will Peter be?
"Magnificent! The truth is, we don't know. I've seen him do 'Doctor-ish' stuff and it's worked. I've seen him deal with the technobabble, I've seen him deal with the nonsense...
"I wrote scenes [for the audition] that were deliberately impossible - deliberately impossible dialogue, just to see, 'Can you do the impossible, even without gunk being poured on you?' and now we're going to pour gunk on you, and throw a lizard at you, and ask you to say all this stuff and explain the plot...
"So we don't know [what he'll be like] yet, we're going to work on that. Matt developed hugely as he approached the part over the first few episodes and we'll do the same with Peter."
What will the 12th Doctor wear?
"Clothes. Anything else would be just really shocking!"
Peter Capaldi will debut as the 12th Doctor in Doctor Who's 2013 Christmas special. The series returns to BBC One for its 50th anniversary special on November 23.