One last time, the cast and creators behind the latest incarnation of Doctor Who gathered together at Comic-Con International in San Diego. At the celebratory panel, they teased some tidbits of the Twelfth Doctor's final appearance in the upcoming holiday episode, and toasted Peter Capaldi, who has played the character for four years now. But beforehand, CBR scored some additional insight at a series of roundtable interviews with Capaldi, his co-stars Pearl Mackie, Michelle Gomez, and Matt Lucas, senior Doctor Who writer Mark Gatiss, and outgoing showrunner Steven Moffat, who'll hand off the series to Chris Chibnall following the special.

First and foremost, the holiday episode will be called "Twice Upon a Time," and according to Moffat will focus on a pivotal moment where the Twelfth Doctor (Capaldi) collides into the timeline of the First Doctor (David Bradley taking on the role originated by William Hartnell) at a pivotal frozen moment. Both are on the brink of regeneration, and resisting it with all the might of the Doctor(s).

"It's one dilemma, one choice, twice," Moffat told us. "It's the Doctor at two moments in his life, willing himself to go on. So it's the Twelfth Doctor telling the First Doctor, 'You've got to change, because you've got lots of stuff to do!' And the First Doctor saying, 'So do you.'"

In a separate interview, Capaldi gushed over the special, which wrapped shooting two weeks back. "(Bradley) delivers a wonderful performance where he manages to evoke William Hartnell, very beautifully, very powerfully. These two Doctors meet at a crucial time in both of their lives, and they have to resolve something together. I don't want to give it away."

Thankfully, Moffat went on to explain that it's through an adventure with a captain -- played by screenwriter (and Sherlock's Mycroft Holmes) Mark Gatiss -- that the Doctors change "the course of what they think."

Some Whovians speculated Gatiss might be playing a young version of the classic Doctor Who character, the Brigadier, who would become a founding member of UNIT (United Nations Intelligence Taskforce, later Unified Intelligence Taskforce). But Gatiss was quick to shoot this idea down. "No, I'm a first World War captain. I can't really say much about it."

He did admit, however, that "the captain, has an impact on the Doctors' lives. It's a beautiful script. I'm very privileged to and thrilled to be asked to do it. It's a sort of chamber piece, a Christmas adventure. It's a beautiful finale for Peter and Steven."

Later at the panel, it was announced that Mackie would reprise her role as the lovable companion Bill for the speical. But in our interviews ahead of this reveal, Mackie was resolutely tight-lipped, only saying she'd be game to return, adding, "It's Doctor Who, never say never."

You can get a glance of her return in "Twice Upon A Time"s trailer:

When Doctors cross timelines, there's a bit of a wild zone feel, as the Time Lords never remember meeting their other selves. As Moffat explained, "When the timelines are out of sync, you can't retain it. Missy said that to the Master, Matt Smith said that to David Tenant (in "The Day of The Doctor"). We have to assume -- we have assumed for years -- that the Doctor only maintains the memories of his most recent incarnation. We have to. Otherwise it wouldn't make any sense. And we can't have Doctor Who not making sense, could we?"

NEXT PAGE: The Christmas Special's Impact on Doctor Who Mythology



The Christmas Special's Impact on Doctor Who Mythology

Asked if "Twice Upon a Time" would have a dramatic impact on the show's mythology like the ret-conning multi-Doctor episodes about the War Doctor had, Capaldi assured us, "It might just be one of those things that Doctor Who does that opens a little door into perhaps why such and such a thing happened. But it's not a substantial change in the canon."

Capaldi also promised that for all its talk of death and farewells, this would be an uplifting holiday special. "It's very nice," he said. "If you're a Doctor Who fan you'll find a lot in it, I think. It's fun. It's about hope and about responsibility. I'm making it sound very grave, but it's actually very funny and festive and joyful.

"But as the best Christmas things always are," Capaldi warned, "it has a kind of melancholic core to it, because it's reflective. It's about time passing, as it does for everyone, even Doctor Who."

Moffat mirrored this sentiment, noting that the plot line of reflection does carry a similarity to the iconic holiday classic, It's A Wonderful Life.  "Its influence is in there," Moffat nodded. "It's not that structure… I did think of doing that, but then thought that Doctor Who stories don't quite work like that. But it's absolutely in there, 'What's the point of me? Shouldn't I just stop?' So it is the idea of, 'I'd like a rest,' well, 'You can't have one.' So the Doctor fell, now he rises, twice."

"But he does have Zsu Zsu's petals in his pocket," Gatiss joked, with a slight guffaw.

As for Gomez and Lucas, they were coy when asked if they might reappear on the series."I can't remember if I'm in it or not," Lucas teased, while Gomez sat silent. Yet, both agreed they're game to return for more adventures. Sure, it seemed like Missy died in the season finale, but Gomez echoed Mackie, saying, "Never say never."

Of course, for all the mystery that remains in "Twice Upon a Time," there is one certainty: it will end with regeneration, and the inevitable introduction of the Thirteenth Doctor, played by Jodie Whittaker.

Each cast member insists they have not met with Whittaker, though Capaldi admitted, "I spoke to her." He told us he wasn't on set for her big scene ("I actually didn't film it with Jodie"), but was full of praise for the next Doctor. "She's great. She's lovely. I think she'll be brilliant. She's very excited it about it all."

Asked if he had any advice for Whittaker as he passes along the torch and the TARDIS keys, Capaldi said he told her, "I don't think she needs any words of wisdom from me. I think she'll be fabulous. I think she'll do her own thing, and it'll be great. Just to enjoy it. That's what I said, just enjoy it."

Doctor Who's "Twice Upon A Time" will air on BBC America this Christmas.