Peter Capaldi's last episode playing the Time Lord premieres on Dec. 25

Frequent Doctor Who writer Mark Gatiss scored a plum role on BBC America’s long-running time travel show 10 years ago playing the villainous Professor Lazarus in a David Tennant-starring tale called “The Lazarus Experiment” and has made a clutch of cameos in the decade since. Now, Gatiss is back on the TARDIS again in a different role, acting opposite Peter Capaldi on this year’s special Christmas episode premiering Dec. 25. What gives? “It’s a bit greedy really,” he concedes. “I’ll have to stop!”

This year’s special Christmas episode, “Twice Upon a Time,” is very special indeed. The show will see the departure of Capaldi, the return of Pearl Mackie’s Bill Potts, a starring role for Game of Thrones actor David Bradley as the First Doctor — a part originally played by William Hartnell back in the ’60s — and the introduction of Jodie Whittaker’s new incarnation of the titular Time Lord. The episode is directed by Rachel Talalay, who oversaw 2015’s beloved “Heaven Sent” episode, and is written by showrunner Steven Moffat, who is himself departing Doctor Who after eight years of steering the show through time and space.

“As we discovered at the climax of the last season, the Doctor is about to regenerate, but doesn’t want to,” says Gatiss. “Not because he doesn’t want to change his face but [he thinks] maybe this is time to finish it. And then unexpectedly, out of the snow, arrives his very first incarnation, now played by the great David Bradley. I play a captain from the First World War who is kind of plucked out of time and then gets involved with the Doctor’s adventures. So, it’s essentially a kind of chamber piece of David, and Peter, and Pearl, and me, and this strange mysterious threat. But really it’s about the wonderful interplay between the two Doctors. Both Doctors are about to regenerate, one of them for the first time, one for the 13th, or whatever it is now. [Laughs] It’s a very charming, funny, and rather moving script, and it was just a privilege to be part of it. I mean, for me, as a lifelong fan, to be in a regeneration story, and to be in Peter’s one, and to be also with the First Doctor, it was amazing. I just had to pinch myself at times. I looked across the set to see two TARDISes next to each other and got a real frisson of old-school Doctor Who-ness.”

Gatiss’ appearance on the show is the result of a personal request made by the departing Moffat, a longtime pal with whom the actor-writer co-created the Benedict Cumberbatch-starring Sherlock.

“Obviously, we’re very close friends and collaborators,” says Gatiss. “Early this year, he took me aside and said, ‘Will you keep the summer free? Because I’ve written a part for you in Peter’s last story and I want you to be there when I go,’ which made me slightly well up. It’s the sort of thing the Doctor says.”

Gatiss has no plans to write further scripts for Doctor Who at present but reveals new showrunner Chris Chibnall (Broadchurch) got in touch about the possibility of him doing so.

“Chris Chibnall very sweetly emailed me and said he’d love me to continue because I’m a big part of the Doctor Who family, which I was very touched by,” he explains. “But they’re shooting Jodie’s first season now. It’s all new people and he has a totally different way of working. So, at the moment, I don’t know. Never say never. But I’ve had such a brilliant time and I feel very blessed to have worked on it for 10 years. It might just be time for someone else to have a go. I must admit, the idea of not writing for it again is very strange. But I’m just very excited about not knowing anything and it’s been a very long time since I’ve been able to say that, literally, when it starts, I won’t know a thing about the new approach, the music, the title sequence, the inside of the TARDIS, the stories, the writers. I don’t know anything! And that’s just like being eight years old and sitting in front of the TV again, and not knowing anything, which in this day and age is a very unusual situation to be [in]. So, I’d just like to enjoy it and have a proper break, and maybe, if Chris wants me, I might do something.”

The Doctor Who special Christmas episode premieres on BBC America on Dec. 25 at 9 p.m. ET. Watch the show’s trailer, above.

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