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

Like former Who boss Russell T Davies before him, Moffat says he’s now leaving Doctor Who writing for good: “I mean certainly in the short term and the short term is very long.

“You can't be the boss of something and then loiter round the office saying ‘Can I do that bit?’ You can't do that can you?” he explained.

“So you have to clear out. I mean I've written an awful lot of it so I don't mind not writing it again. That's not some horror of it, I loved it. I absolutely loved it, I still love it, I still get very excited about it. But I am fine with not writing it again.”

16. And Pearl Mackie never got to realise one of her ambitions either

Meanwhile, departing companion Pearl Mackie has her own offbeat regret about her time on Doctor Who – that despite her character Bill becoming a Cyberman in the series finale, she never got to try the outfit on herself.

“I just wanted to get inside the Cyberman costume, but I was too small,” she said.

“No, it was a real shame. I was like 'Oh my God this is gonna be so exciting,' I was preparing my Cyberman voice for the read-through. And then our script editor just read it, and read it very well I must say, but I was then like....'Oh, OK.’

“And then they wouldn't let me get inside it either, so I was like 'oh'.”

Cyber-fingers crossed she finally get her chance in the Christmas special. After all, there were Mondasian Cyberman in The Tenth Planet…

17. Despite everything, apparently the finished episode isn’t that sad

Sure, the Twelfth Doctor is leaving us forever this Christmas – but that doesn’t mean the entire episode is an absolute blubfest.

“The final episodes of series ten are obviously very dramatic, and quite grim actually,” Gatiss told us, “and this a postscript in a way, but it's lovely.”

“We've done all of the big angsty finale one and now we've got two silly old Doctors talking nonsense at each other,” agreed Moffat.

“It's really funny as well, which is great,” Mackie added. “You need that on Christmas Day after a few glasses of wine, don't you?”

“It's got a great story and Steven said before we started the read-through, we sat round and he said just ‘Make it fun, have fun.”” Bradley said.

“Keep it light and keep it moving because it's not only a Doctor Who episode, it’s a Christmas episode. There's a kind of an It's a Wonderful Life kind of feel to it and I think that's what they want and they wanted me and Peter to have fun.”

18. Though the crew still got emotional sometimes…

Even amongst the laughter, there apparently was a palpable sense that something was coming to an end – especially through the eyes of relative new boy on set David Bradley.

“Steven [Moffat] was around a lot of the time and we'd hardly started shooting before we were having selfies with Steven, we'd make sure we've got the memory in the bank,” Bradley recalled.

“So there was a kind of sadness to it; but at the same time we felt ‘Oh, this could be really good.’”

“Steven said to me the other day, he was waiting for the moment where it felt like 'Oh, this is it.'” Gatiss added. “But it's just the same. I don't think it'll actually hit anyone for quite a while. Because you're sort of a bit numbed about that.

“There'll probably come a day when Peter probably has to do one ADR line of a word like 'Oh'. And that'll be it. And then the door clicks.”

19. And finally – the cast think the special is an absolute belter

“I think it's a very fitting and lovely ending for the both of them,” Gatiss said of the finished episode’s legacy as the last story involving both Capaldi and Moffat.

“I think it's a fantastic episode, I think it's going to be very exciting,” Mackie agreed.

“I am straining at the leash to see it, to see what we've done,” Bradley concluded. “It’s best to go out with a bit of a bang.”

Sounds like we can expect a total Christmas cracker.

Doctor Who: Twice Upon a Time airs on BBC1 on Christmas Day (Monday 25th December) at 5:30pm