In this special interview, Moffat talks about resurrecting Gallifrey, a potential return for the Zygons and Billie Piper's unorthodox reappearance - and gets effusive about Doctor Who's worldwide success!
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How did you feel after 'The Day of the Doctor' finally went out across the world?
"I was in a state of terrible tension [beforehand] - it's terrifying to go and sit in the middle of an audience, watching your show - this show in particular. I was so tense... still tense when I went onto the BBC Three show - quite obviously so! I had to drink quite a lot of wine to get through that!
"But slowly - with the amazing reviews and the extraordinary overnights… - I realised that it has been an extraordinary success at every level.
"You simply can't understand the amount of relief. It was an amazing feeling."
Why did you decide to bring back Gallifrey?
"It was about a year ago, I remember thinking, 'What occasion in the Doctor's life is the most important?' - well, it's the day he blew up Gallifrey.
"Then I tried to imagine what writing that scene would be like and I thought, 'There's kids on Gallifrey and he's going to push the button? He wouldn't!' I don't care what's at stake, he's not going to do it.
"So that was the story - of course he never did that, he couldn't. He's the Doctor - he's the man who doesn't do that. He's defined by the fact that he doesn't do that, whatever the cost, he will find another way.
"So it had to be the story of what really happened, that he's forgotten. Of course he didn't - he's Doctor Who! He doesn't do things like that!"
How will the search for Gallifrey figure into future Doctor Who?
"He has the possibility of going home - he can find Gallifrey - but it might take him a while, who knows? And who knows what he'll do when he gets there? Get bored and run away again, I would think! But he has a mission statement.
"It was fascinating when Doctor Who first came back that he was this war survivor dealing with guilt and rage - that was his story. Of course, he slowly gets over that and then there's a danger that he just becomes about... farting about a bit, which starts to take some of the baseline out of the show somehow. So we've given him something to pitch for."
"It's not like we'll spend every episode saying, 'I nearly found it!' - we absolutely will not do that - but it gives him somewhere to go. The Doctor doesn't know he's a character in a television show - he doesn't know he's having adventures for our entertainment - he's got to have something to do and that will be the thing he does."
Why did you decide to reintroduce the Zygons in the anniversary special?
"Because I can! I just thought... wouldn't it be cool if we had some Zygons? I'd like to use them again in a story actually because everything is a support to the Doctor in that particular show - it's not really how you'd do a Zygon story.
"But I love them - they have a cool sci-fi name and they look great. We made very little change to the original design - they're just beautiful things."
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'The Day of the Doctor' revisited notions from the Russell T Davies era, like the Time War - did you discuss the episode with Russell?
"It's difficult with Russell 'cos he doesn't like spoilers. When we worked together on the show, we tried not to spoil each other. In general, he doesn't want to know.
"He always gets in touch after an episode airs to enthuse away like a fanboy. In years to come, when it's not me [as showrunner], I'll hand it all over and I will not want to be spoiled."
Why did you decide to have Billie Piper in the special, but not actually as Rose?
"I thought that the story of Rose - which is beautiful - was done. I didn't want to add to it and I didn't feel qualified to - that was always Russell's story. But we did want Billie.
"The way Russell ended it - going back to just before [she met the Doctor] was perfect. I didn't want to stick another bit in - that would be wrong. But we wanted a way to get Billie Piper - one of the absolute heroes of Doctor Who - back in the show, without interfering with the story of Rose Tyler. I might've spoiled something!"
How do you feel now that Doctor Who's 50th special has been simulcast across the globe?
"I mostly think… Doctor Who did that - the show that I love, the show that was my favourite when it wasn't cool for it to be your favourite. That show has done that. Sod you, Star Trek fans! Look! My show - the one you laughed at me for in the playground for liking - it did it. It's bigger and cooler than your show - that's what I think.
"I don't think that as showrunner - the show I'm a fan of is definitely cooler than you and your Mum and Battlestar Galactica and everything!
"It's one phenomenal British success, isn't it? We really nailed this one, didn't we? I mean, properly - come on! I don't think in Britain we're terribly good at… it's not boasting, it's just… that was tremendous, that was terrific.
"So much of the time - a bizarre amount of the time - the British press and fans try to prove that Doctor Who is less successful than it used to be. So much of their time! Even though the numbers right in front of their little eyes tell you the exact opposite story - even before 'The Day of the Doctor', the numbers tell you the exact opposite story!
"People say it's not been as successful since David Tennant left. It's gained about 70 million viewers, but if you think that's less successful, you might want to go back to school and pay particular attention to arithmetic!
"It has grown and grown and that's not me boasting about me - that's me boasting about an amazing show. It's a great and wonderful and magnificent achievement and we should say it. The Americans would say it - they'd say, 'We've done this well and we're proud of it'. Well, we've done this well and we're proud of it!"
Doctor Who will return to BBC One this Christmas.