In a special souvenir issue of Radio Times magazine, Steven Moffat previews his final Doctor Who episode – Christmas special Twice upon a Time, starring departing 12th Doctor Peter Capaldi and David Bradley as the first Doctor. He considers the future of Doctor Who and his own career and reveals some juicy nuggets of information from his eight years on the show.

Below are some excerpts from the interview – to read the whole thing, including Steven’s thoughts on more Sherlock and his upcoming new series Dracula, get the new issue of Radio Times, on sale from Saturday 2nd December.

Patrick Mulkern: Let’s kick off with Twice upon a Time, your final Christmas special. How would you set it?

Steven Moffat: We ended the last series with Peter Capaldi’s 12th Doctor about to regenerate and refusing. He’s had enough of becoming other people. A subject he expands on in the special. He’s having a strop. In a wintry landscape he meets the first Doctor [David Bradley taking on the role from William Hartnell, who died in 1975], who is also refusing to change.

But how can the Doctor put his regeneration on hold?
We’re going with the idea that it is at some level voluntary. Remember the John Simm Master refused to regenerate at the end of The Last of the Time Lords [in 2007]. So you have to commit and choose to change rather than die. In the bonkers science of regeneration, gender doesn’t seem to be a problem, but what has always puzzled me is: how does he change height? That means between Colin Baker and Sylvester McCoy a hell of a lot of matter goes missing from the universe. Where is it?

Indeed! OK, back to the special…
It starts with a reprise of The Tenth Planet [the first Doctor’s finale from 1966]. We do the “Previously on Doctor Who…” and follow the first Doctor out of that adventure, having left his companions Ben and Polly behind. So then it’s the 12th Doctor telling the first that he has to regenerate, and realising he must as well. This is the moment where he decides whether or not to go on. And it will mark the only time that David Bradley has played a younger version of Jodie Whittaker.

But it’s not just about two old men dying. You’re making this one more comedic and fun.
Yes, we got to a very dark and angst-y place at the end of the last series. This is Christmas Day so we’re not going to have an hour of two suicidal Doctors. That’s not appropriate for Christmas Day or Doctor Who. There’s a tradition of the Doctors being funny when they get together. When Doctors meet, it’s a laugh. And I suppose at the back of my mind I’ve known for ages the next Doctor was going to be a woman – although I didn’t know which woman – so I was thinking, “Why does he subconsciously make that choice?” Maybe seeing the whole span of his life as a man, seeing himself as the Hartnell Doctor, might make him think maybe it’s time to be a bit more progressive. Looking at how the first Doctor was, he’s hilariously not progressive.

Without being too outrageous I think we have re-created that version of Hartnell’s Doctor, with all the 1960s political incorrectness in place. At the same time the original Doctor has a lot of fun at the expense of the modern one’s sonic glasses and electric guitar. There’s something funny about the 12th Doctor realising that he came from this politically incorrect, funny old man. This is who he was.

Were the Tenth Planet scripts I gave you useful?
Yes they were. We don’t use all that much in the finished show and the trouble is most of those actors didn’t stick rigidly to the script anyway. Michael Craze and Anneke Wills [companions Ben and Polly] improvised a lot of it and it’s better. As you know in the original script there’s a line they dropped where the Doctor is resisting his regeneration. It’s currently in the special but we might drop it because it makes it slightly different from the scene at the end of The Doctor Falls as it was shown.

The sets from The Tenth Planet – the Snowcap polar base and the Cybership – have been impressively re-created [above].
In the finished cut you only glimpse the polar base. The only thing that we do more or less in its entirety is when Ben and Polly get the Doctor out of his cage on the Cyberman spaceship and he says, “It’s far from being all over.”

So you’ve cast Mark Gatiss in it and Toby Whithouse [Doctor Who and Being Human writer] as First World War soldiers.
I asked Mark a long time ago to make sure he’d be available and then I needed another actor to lie in a bomb crater and talk in German. I thought, “Wouldn’t it be great if we got Toby?” who is also a very fine actor. So we have the two principals of my writing room sitting with guns drawn on each other.

Were you tempted to plonk Chris Chibnall in there as well?
He and I are not actors so we shouldn’t be in it. There’s a possible world where we could’ve been corpses if only we had the time. And those two are terrific actors.

Why does the first Doctor lend himself to being played by other actors more than any other Doctor?
A weird thing happened with the first Doctor. In The Five Doctors [1993], which I love, we didn’t really remember what Hartnell was like. Somebody else turned up who didn’t look like him or behave like him, and wore a strikingly different costume but was close enough to the general silhouette. Richard Hurndall was rather good and very engaging, but the fact is he was not the Hartnell Doctor.

But even before that during the mid 60s, when Hartnell was still the Doctor on TV, we had Peter Cushing in the two Dalek feature films playing a version of that grandfatherly Doctor.
That’s different. I adore Peter Cushing but I think, outside the charmed circle of the insane – and I mean US and those like us – people don’t know that he was the Doctor. They know he was Sherlock Holmes, they know he was Van Helsing. They don’t know he was Doctor Who.

I’m sure at the time they did. They were very popular films in their day. So it was acceptable even in the 60s that you could do that with the character.
Yeah but that happened with Quatermass too. And when something jumps medium, you’re more likely to accept a recast. And they rebooted it and made Cushing’s Doctor human. By the time they made The Five Doctors, enough time had passed and we weren’t so aware of him. I have to say they made an incredibly classy decision to have a clip of William Hartnell introducing it. It weirdly sanctified Richard Hurndall taking on the role. As if to say: “We’re not kidding you on, we’re not pretending it’s the same man. We acknowledge the original.”

So much so that in this Christmas special when it starts we have Hartnell and the original companions and then it becomes David Bradley. So we say, “Here’s the real original one. Here is our one and, oddly enough, don’t they look similar?” Anneke’s character Polly is in it and Anneke herself is in it with the line, “Have you got no feelings?”

How far back did you plan to bring back the first Doctor? In the last series there seemed to be lots of references. For a start you named the companion Bill and her girlfriend was Heather like Bill Hartnell and his wife Heather.
Oh, that was a happy accident. Absolutely accidental.

So you didn’t name Bill after Hartnell – or even after your father, Bill?
No, if she’s named after anyone it’s that I liked the fact that, when we were doing The Day of the Doctor [2013], David Tennant called Billie [Piper] Bill. I thought that was a good name for somebody so I logged it then.

I wasn’t especially building towards the return of the first Doctor. Available on videotape is the exact moment where it became possible. At the New York ComicCon last year someone was asking me about which Doctors I wanted to be in The Day of the Doctor. And the Doctor you’d really like to meet the modern Doctor is the William Hartnell Doctor because he’s moved on so much, because that would be the entire span of the character’s life. The first Doctor would be so shocked that he is going to become this strutting megalomaniac. And so I said, “But we can’t do it,” and then Peter said, “We could get David Bradley.” “Oh yeah…!”

Pearl Mackie is back as Bill – are you surprised how popular she’s been?
No, you know a star when you see one. She’s great. People responded strongly to her and they like the contrast between her and Peter. They liked Nardole and the little family unit together in the university. You could have watched them for five years in that set-up. And seen more of his lectures and the scenes in my head I never got to write, which were the Doctor taking his role in university very seriously and going to budget meetings and arguing for a new science block. He’s not a pretend professor; he’s a real one and he’s got serious views about his job.

Is that your early years as a teacher coming to the surface?
No, not really. I was a very indifferent teacher and would have run away as soon as I could in my time machine. That was a lifetime ago anyway. I left in the late 80s.

Have any of your students ever got in touch with you?
Rarely. I’ve met a couple. Of course they seem about the same age as me because I was a young teacher in my early 20s and a lot of the ones I’d remember were then in their late teens.

Doctor Who has a knack of spotting talent, actors who are just starting out, like Andrew Garfield who was in Daleks in Manhattan [2007] and now has a huge Hollywood career and was in Angels in America.
Our casting director Andy Pryor is so assiduous on who’s coming up. We’d never have seen Matt Smith but for him. We had Olivia Colman [in 2010] a heartbeat before she became a goddess. She was already regarded as a genuinely great actress but within a year of that she was the darling of the nation.

It was the same with Carey Mulligan as Sally Sparrow in Blink (the Weeping Angels’ debut and Radio Times readers’ favourite episode).
Oh my God, Carey Mulligan! It’s funny but Blink, I say immodestly, is a very famous episode of television and yet Carey Mulligan, who was the star of it, I’m almost certain wouldn’t even remember being in Doctor Who. I don’t think she was much of a fan, or anything. They liked her so much, they said, “Do you want to be the next companion?” but she said no. God, she was amazing.

So you’ve never tried to get her back in any capacity?
No. I know it’s a no.

What a shame!
I agree. But then does that character become more special because you never see her again? She just passes through the Doctor’s life. It’s surprising the people who do love being part of it. John Hurt loved being the Doctor and was quite insistent, asking, “Am I a real Doctor? Do I really count?” And we said, “Yes, you count. You’re on the poster. It is definitely you.” David Bradley is so thrilled that he is really the Doctor now. Because obviously he was sort of the Doctor in An Adventure in Space and Time [2013]. He’s been a star for ever, but being the Doctor is special, somehow. He’s a bit like Hartnell in a way. He’s got a mean face but he’s the nicest man alive, so sweet and generous.

Doctor Who: Twice upon a Time will air on BBC1 on Christmas Day