No one is telling what will happen in this year's Doctor Who Christmas Special, but if Radio Times can tease a hint out of anyone, it will be the stars: Matt Smith, Alexander Armstrong, Bill Bailey and Arabella Weir talk about what they're up to.
"There's a Narnia-esque shape and feel to the telling of this story," says Matt Smith, teasingly, of the Doctor's seasonal outing.
"Whereas last year felt more like a Christmas romp, there's a slow-burning, ethereal magic to this. We've covered a whole forest in snow. The scale is vast and there's just something wonderfully magical about it because it's never that snowy in this country, except maybe in Scotland... and on the telly!
"It does it for you: all the snow and the lovely smell of the pine trees. I'm really, really looking forward to Christmas now."
Can he confirm that the story is set during the Second World War? "Yes, but not on the frontlines," explains Smith. "The Doctor meets this plucky, feisty lady called Madge and finds out something very tragic about her life.
"So he invites her and her children back to his house to give them the best Christmas ever, which turns out to be the best Christmas ever in an extremely dangerous forest."
Of course, what every Whovian is eager to know about is the monster. "I think it's going to be one of our best," says Smith, discreetly. "The design is classic."
He's much more expansive about the stunts: "I've been doing all my own. One involves a huge explosion: I had just three seconds to jump back from this giant fireball. Believe me, the fireball does so much of the acting for you! It was only afterwards that I realised I could have been seriously charred."
He laughs, having evidently relished his feat. "I've been really enjoying it. Hopefully we'll see more action/adventure-y Doctor next season."
Alexander Armstrong, just one of many familiar faces RT bumped into when we visited the set in Cardiff, makes his Doctor Who debut - in the flesh (fans will know he voiced the computer Mr Smith in The Sarah Jane Adventures and two Who episodes in 2008).
He plays an RAF pilot - "which is a bit of a leap of faith," he jokes - and recently found himself in a Lincolnshire airfield at the helm of a Lancaster bomber. "They're very claustrophobic," he confides. "By the time I'd climbed over various bulkheads to get into the compartment, I was dripping with sweat.
"It was roasting in there because there's no ventilation. I don't know how they survived the war! I had to hang out of the pilot's window gulping in air." Fortunately perhaps, Armstrong's bomber never actually left the ground.
Devotees of Armstrong's sketch show will know this isn't the first time he's been a Second World War airman, but he insists the similarities end there: "I'm keen that he shouldn't be a carbon copy of The Armstrong & Miller Show pilot. He's a bit more, erm, normal."
What does his comedy co-pilot Ben Miller make of Armstrong netting the role? "I told him yesterday, feigning casualness, but I haven't dared tell him what I'm playing - he can read about that in Radio Times! It's rather like telling your wife you're going off for a weekend with your mistress."
Bill Bailey is less coy about his Who debut. "It's the equivalent of a knighthood," he chuckles. "I've watched Doctor Who for as long as I can remember. My earliest memory of watching TV is hiding behind the sofa from the Cybermen. I had an irrational fear of leaving doors open for years after that."
"The reaction is extraordinary. I mentioned on my Twitter feed that I was going to be in the Christmas special and the whole thing just lit up like a Christmas tree with people clamouring to know what I was playing." The most popular suggestion: an Ood.
So who or what is Bailey playing? "I'm worried that I'll be tackled to the ground by the BBC spy team if I reveal any spoilers," he protests, only half-kidding. However, the comic does eventually reveal that he's a "futuristic forestry commission worker", along with Arabella Weir and Paul Bazely. "We're like space gardeners... we're jobsworths, really."
But are they goodies or baddies? "We look very frightening when you first see us, but when our helmets come off it becomes clear we're not," says Weir. "We're more like the Three Stooges."
Arabella Weir is one of only two women who can claim to having played the Doctor - the other being Joanna Lumley. In 2003, Weir starred in an audio play in which the Time Lord regenerated as a woman - a rather unladylike one.
"The Doctor basically came back as a woman who worked in Sainsbury's," explains Weir. "She burped, farted and swilled pints beautifully."
On the behind-the-scenes feature that accompanied the play, Weir happened to mention her lodger at the time - a little-known actor called David Tennant: "They asked if I was a long-term Doctor Who fan," recalls Weir, "and I said: 'No, I'm only doing this because my best friend David Tennant's a big fan of it.'"
Two years later Tennant became a household name after also being cast as the Time Lord.
Filming the special
Unlike Matt Smith, Weir and co will not be performing any stunts because once in full costume they can barely sip a cup of tea ("You have to get someone to lift it"), never mind evade a fireball.
"For the night shoot we had to be driven to the location in a people carrier and that was the biggest challenge by far," says Bill Bailey. "I had to sort of roll in, face down - we didn't look like an elite fighting force at that stage."
What on earth do they wear? "There's a gaming element, a slight insect look and a bit of agriculture in there, too," offers the comedian enigmatically, then seals his lips - leaving the rest up to our imagination.
Doctor Who: The Doctor, the Widow and the Wardrobe is on BBC1 and BBC1 HD at 7:00pm on Christmas Day
This is an edited version of an article from the issue of Radio Times magazine that went on sale 26 November 2011.