Peter Capaldi as the Doctor and Matt Lucas as Nardole in Doctor Who: The Return of Doctor Mysterio. Credit:Simon Ridgway We were invited to play with the knobs on the console. We fiddled with the clockwork bit, the squidgy alien bit. I found a chunky switch that made a dial flick right, then left. Dial goes right. Dial goes left. Dial does nothing any more. Oops. If the Christmas special is about the Doctor trying to fix a small but vital switch on the Tardis, you know who to blame.

Except it's not, because it's about superheroes. "I think the Christmas special… which is the only [Doctor Who episode] this year… I think is one of [head writer] Steve [Moffat]'s best ever scripts, absolutely beautiful," Moffat's occasional collaborator Mark Gatiss said in September. "It made me cry." Moffat jokes: "I think he got his finger trapped in a door". But when Moffat's wife read the script – his seventh Doctor Who Christmas special – she also cried. He was more than a little surprised.

"I just thought it was rather good fun," he said. "She said it was so sweet and lovely, I said 'what are you talking about?' It's very emotional, apparently. "There's always a love story at the heart of a good superhero story. It's always a love triangle for two, isn't it? I love that. The only superhero thing I ever want to write is the Clark Kent Lois Lane story, when you can't own up to being the man she's in love with." He laughs. Minor spoiler alert: the Doctor arrives in New York on Christmas Eve, accidentally confers superpowers on a comic book loving child, then many years later has to deal with the result: a man who's "putting on the rubber and flying around". "The best thing about superheroes is not the superhero, it's the guy he pretends to be the rest of the time," says Moffat. "It's the secret identity… Clark Kent is actually the main character. Lois Lane is the other main character and Superman just does the second unit stuff.

"More or less any tall handsome man can play [Superman] if they're prepared to fold their arms long enough. It's playing Clark Kent that's the key. You love Clark Kent because he has to pretend he's not a God all the time." Moffat says Doctor Who is definitely not a superhero. "A lot of the fun of writing the Doctor is that really he's a sort of charlatan. He pretends to be much more powerful than he is and bluffs his way around the universe… but we all know he can't even drive his own time machine properly. He's your crazy uncle who's got his own time machine and a lot of cheek." Speaking of which, Peter Capaldi is having a great time. The Doctor Who Christmas episodes have a "responsibility to be festive", Capaldi says – and this year's fulfils the brief. "It's full of very ironic gags and comic book gags," he says. "It's very enjoyable."

It's been too long between Whos, he says. He wrecked his knee chasing Zygons in the last series and needed an operation. But he had to finish the series first, literally limping to the finish line. He leans back, like an eccentric, genial Scottish uncle, with a cheeky, toothy smile. "The doctors always say 'oh you'll be running around in two weeks' but you never are," he says. "It was about four, five weeks before I could hobble around on a stick – which I loved. It was rather theatrical. My wife had to stop me buying one with a gold demon's head." Because of his knee he couldn't work, not even a play to fill in the time between series. "I was going stir crazy," he says. "And the gap kept opening up, we were supposed to come back much earlier but for whatever reason it got bigger and bigger. I was dying to get back again." He was half-hoping the long pause would give him some respite from fans.

"The funny thing is I seem to be recognised more now even though the show's not on – I can't figure out how that works," he says. "Some days you'd like to just quietly pop to the shops and not have conversations with people. "Doctor Who's exciting but I'm not, so I get into a panic because I think I mustn't disappoint them - but I don't know what to do." He hasn't decided whether the 2017 series will be his last. It's not that he dislikes being an "ambassador" for the series. But it weighs on him. "I had my 25th wedding anniversary this year and we had a party, and I had a band, and I suddenly realised: I danced. I think it was the first time I've danced in years because I knew the party was just full of my friends who don't care how Doctor Who dances."

On the other hand he loves the escapism of the series, the immediate fairytale element. "There is something potent about the death motif in it," he says. "People get very fascinated by the fact that the Doctor can be extinguished. They bond with him, then he has to go through this (regeneration). There's something deeper in this." WHAT Doctor Who: The Return of Doctor Mysterio WHEN ABC, Monday, December 26, 7.30pm Some Who Christmas facts - One of the show's producers – Peter Bennett – oversaw Christopher Reeve's flying scenes in Superman.

- Some of the episode was shot in Bulgaria, where a film set contains a replica of two New York blocks. - The Tardis hasn't been completely renovated, but it does have a new "flight" mode with extra flashing lights. - Judging by the books on the shelves inside the Tardis, Doctor Who (or one of his companions) is a Robert Harris fan. - The Doctor's new companion, played by Pearl Mackie, is not introduced in the Christmas special. The new series begins next year: Moffat says the first episode is a "big family friendly action-based spectacular… it's a reboot to Doctor Who at its simplest purest form". - 2017 will be Steven Moffat's last as head writer – he's handing over to Chris Chibnall, who has worked on Who, Torchwood (as head writer) and created the hit detective series Broadchurch.