19 things we learned from the set of the Doctor Who Christmas special

As might be expected in an episode that brings back the very first incarnation of the Doctor (as well as his 1960s Tardis), the special has a visually similar feel to episodes from the classic run of the series that ran from 1963 to 1989 – or at least according to lifelong Who fan Mark Gatiss, who plays a First World War Captain in Twice Upon a Time.

“There’s the old Tardis, in these ruins with a red gel on it,” Gatiss explained. “And it's just like [1975’s] Planet of Evil, or [1976’s] Brain of Morbius. I could take a picture, make it into black and white and send it to a fan saying 'What's this from?'

“It's like one of those big empty shots of TV centre with the Tardis in. And you just go 'It's exactly the same.’ It's full of manifest joys.”

He added: “I don't think it was meant to look like Planet of Evil, it just does!”

Old-school Whovians – time to compare and contrast.

Peter Capaldi is a seriously dedicated explosion-dodger

While RadioTimes.com was on set, we got to watch a scene between Peter Capaldi’s Twelfth Doctor and David Bradley’s First Doctor, as the pair dodge small explosions in the ruins of an alien planet.

And because these are real explosions, courtesy of Real SFX’s Danny Hargreaves, the actors need to be very careful to hit their marks and avoid them, all while delivering their lines – which might explain why outgoing Doctor Peter Capaldi spent a good 20 minutes alone in a corner practicing his lines and footwork (yelling “Bosh!” to signify the explosions) ahead of the final take.

“It’s very exhausting! Very exhausting – like dancing,” Capaldi told us after his self-imposed exile was completed. You have to admire the dedication.

Meanwhile, Bradley merely quipped: “I’ll leave all the leaping to Peter.”

The production team almost took the opportunity to fix an old plot hole

Of course, bringing back a long-departed Doctor offers some opportunities to clear up inconsistencies from the original series, when Doctor Who’s rules and continuity weren’t quite worked out yet.

Take, for example, the fact that Hartnell’s Doctor appears to change his clothes along with his body when regenerating into Second Doctor Patrick Troughton, a feat inconsistent with later versions of the process and a plot hole that Moffat admitted he was tempted to fill in.

“I nearly went as ludicrously as explaining the clothes change,” Moffat said. “But then I just thought no, people are paying their license fees for other reasons than this. So that one’s still out there."

You can read more about this attempted repair here.

Pearl Mackie thinks there’s “more to explore” with Bill

While this episode is sadly the last we’ll see of Pearl Mackie’s companion Bill Potts in Doctor Who, the actress is open to returning, and says there’s plenty more she’d like to discover about her character.

“I think you can never explore enough of any character,” she said.

“I think one of the things about Bill is that she's very real. She's very human – and with any person, there's always more to explore. It's not like 'Oh OK that's great, that's it, that's my book, I'm done.'"

“So yeah, I think there's room for plenty more exploration if that happened."

You heard it here first – the campaign for Bill’s Doctor Who return starts now.