Last month, the BBC announced that Doctor Who star Matt Smith had decided to leave the 50-year-old British science fiction show after this year’s special Christmas episode. So how goes the search for the next Doctor? “Well, it’s always just terrifying,” says Doctor Who executive producer Steven Moffat, who recruited Smith to replace his predecessor, David Tennant, back in late 2008. “If you’re a Doctor Who fan, as I have been all my life, you’ve been doing fantasy casting for this part for as long as you can remember. But when you’re suddenly faced with the reality that you are going to sit there and you are going to make that decision it does feel absolutely chilling. There’s a very big range of people who could play it and different ways you could go with it. We must get this right. One false move and the show’s over.”

Below, Moffat — who will be appearing at the Comic-Con Doctor Who 50th anniversary panel along with Smith and other Who notables — talks more about the search for a new Doctor, Smith’s decision to leave the show, and why he’s got a bone to pick with Ryan Gosling.

ENTERTAINMENT WEEKLY: When do you hope to announce the identity of the new Doctor?

STEVEN MOFFAT: Unless we have an insane plan, we’ll announce a new Doctor within days of finalizing the new Doctor. Because it’s very very hard to keep any kind of a secret. The last time, when we chose Matt, we had to hold over on that one, because there was a Christmas Doctor Who special called “The Next Doctor” for which Russell (T. Davies, former Doctor Who showrunner) was playing the game of pretending it was going to be David Morrissey. So we couldn’t deflate that. [Laughs]. But I think we’ll go public pretty fast.

Have you at least narrowed down the sex of the actor who will be playing the new Doctor?

I’m not going to comment at all on the direction we’re going. Sorry!

Are you hoping the new Doctor will appear in this year’s Christmas special?

Yes. That’s not the hope — that’s the plan. It’ll be the traditional regeneration. You know, the eleventh will fall and the twelfth shall rise. And you’ll see that in the closing moments of the show. I mean, you sometimes sit and think, “Are there better ways of doing it? Is there a different way of doing it?” But quite honestly what could be better than that? It’s just too exciting. [Laughs]

Is Matt going to have to wear a wig when he films the Christmas special? He seems to have had a very severe haircut for his role in Ryan Gosling’s How to Catch a Monster.

We’re sprinkling fertilizer on his head as we speak. I don’t know. If you care to take a look at “The Angels Take Manhattan” there are a couple of scenes that Karen Gillan came back to do in the graveyard after she’d had her radical haircut and she is wearing what seems like a strategically draped otter on her head. [Laughs]

However, we effect it, the Doctor will turn up in his trademark coif. We can’t have Matt’s last stand in the TARDIS without his proper look. So, thank you, Ryan Gosling…

Am I right in thinking that the new series—the first post-Matt shows—will be broadcast in late summer 2014?

I think that’s probably right. But these things change so often.

How did you find out that Matt was leaving?

Well, I’ve known broadly speaking for a very long while because I knew how long, when he first came in, he was broadly speaking intending to do. And obviously, being the man I am, I always tried to persuade him to do longer and to do more and he [stayed] a little bit longer than he intended to. I knew that he would do what most of them do and do his three years. It’s a difficult thing for any departing Doctor.

Curiously enough, it was really to me that David Tennant resigned. Because he was considering whether to continue now that I was taking over. And both of them went through he same experience. It’s not like leaving any other part, it really isn’t. It’s sort of like abdicating [the throne] and it’s genuinely emotional, it’s upsetting. It’s an upheaval in your life. It’s something you really have to contemplate. And I remember what both of them said during their period of anguish — when they were contemplating letting somebody else into the TARDIS — they both said, “There’s part of me, I would just stay doing it forever. So, if I don’t leave now, maybe I’ll just carry on forever. And that wouldn’t be right for me or the show.” It was a difficult, emotional experience for both of them.

When did he actually tell you that he was leaving?

We discussed ages ago that we would do three series and then he would do the 50th and then he’d do Christmas. That was Plan A for a very, very long while. That may sound cold that it was so far in advance but you’ve got to plan a career. [Laughs] The question was, “Will I be able to talk him out of it?” We went out for lunch and he said that he’d come very close to doing another series but it was the same argument: “If I do another series, I think I might do two more series, or three more series. I think I might never leave.” It’s that thing of wanting to leave while you’re a huge hit and not let it tail off. It’s part of the ecology of the program, it’s part of the DNA of the program, that there is going to be a new Doctor now and then.

None of them ever want to outstay their welcome, and Matt certainly didn’t. Not that I think he was in any danger of that, frankly. It’s also, it has to be said, an overwhelming schedule for the actor playing the Doctor. As a workload it precludes you doing anything else. It precludes theatre, it precludes any significant other television or film work really. And even trying to crowbar some time in for him to do other things — which was part of our charm offensive — in the end he wanted to go and develop the other parts of his career.

What can you tell us about this November’s Doctor Who 50th anniversary show?

[Laughs] Oh, well, very, very little. It will feature of course Matt and Jenna Coleman, but in addition there’ll be Billie Piper and David Tennant and John Hurt. But we’ve been really quite careful. We have a philosophy that anything we shot outside we had to own up to but the rest of it…You’re just going to have to wait until November to find out about.

What is the format of the 50th anniversary special? Is it movie-length?

It’s a special episode. I think you could call it movie-length, yeah. I mean, I’m saying that with a slight hint of vagueness because I don’t know the finished running time. [Laughs] It’s certainly well over an hour.

What was it like having Matt and David together?

They really loved each other and had a huge laugh together. And of course they’ve been through this experience that only the two of them can talk about, really, in the modern world. They are the two people that have played that part at a time when the series is this big. They spent the entire time just sitting together talking animatedly.

Could you talk about how their Doctors relate to each other? Traditionally, there’s been a bit of edge when Doctors have met each other onscreen?

Well, when you’re talking to yourself, there’s no filter! You don’t spare yourself! They’re quite a fun pairing, I would say. There’s a bit of the normal joshing of each other but they’re both such enthusiastic Doctors. While they might be sort of competing slightly, they’re both standing there saying, “Oh god, it’s so cool, there’s two of me!” So, it’s very different. I think the other one that worked brilliantly was Jon Pertwee and Patrick Troughton. They were incredibly funny together. This is very different from that but it’s a sublime double act.

What was it like working with John Hurt?

He’s wonderful. That’s hardly a headline: “John Hurt is a very good actor.” But he’s terrific. It’s a lovely 50th anniversary treat, I suppose. You get a whole new Doctor played by a proper screen legend.

The Tom Baker-era monsters the zygons are coming back?

Oh yes, that’s confirmed. We had to do a scene with the zygons outside, so there was no point in pretending they weren’t there. We kept very close to the original design. It’s a cracking monster.

I grew up in the ’70s and the zygons loom very large in my memory. I was quite surprised to recently discover they only really appeared in one episode.

That’s right. Only one story. Despite the fact that they are clearly one of the most successful monsters the show has ever had.

The other main part of the 50th anniversary celebrations is the TV movie An Adventure in Space and Time, which details the creation of the show and actor William Hartnell’s tenure as the original Doctor. What else can you tell us about that?

Oh, it’s gorgeous. It’s a very, very different celebration of Doctor Who.

Are you aware that David Bradley (who plays Hartnell in the movie) is now a thousand times more famous in America than he was a couple of months ago thanks to his pivotal role on the recent season of Game of Thrones?

Oh, brilliant. That’s excellent. I love David. He’s such a clever actor. And we’ve had him in the proper show as well. He’s in “Dinosaurs On a Spaceship.”

How much longer do you yourself intend to stay with the show?

I think a year at a time. I’ve signed up for this next year, with the new Doctor. It’s one of those jobs when you know when you’ve had enough. At the moment I haven’t had enough and I’m thoroughly enjoying it. I’m very excited for the challenge of the new Doctor and establishing that new Doctor. So, no plans to leave as yet. But that doesn’t mean I’ll be here for 20 years. There will come that day when I think it’s time someone else had a go and it’s time I did something else.

You’re also the executive producer of Sherlock. Have you finished shooting the new series yet?

Oh, I wish! We’ve done two. But we’ve now got a small gap — a small gap? A large gap! — while Martin (Freeman) goes back to New Zealand to film a bit more of the Hobbit and then he’ll return to us. Hopefully, by that time, I’ll actually have finished the Sherlock script I’m writing and we’ll make another one.

We’ll, I’d better let you go off and do that.

[Laughs] Yeah, that might be an idea.