This Christmas will mark a major turning point for Doctor Who. Peter Capaldi, who has played the Twelfth Doctor for three seasons, will make make one final appearance as the titular hero in a seemingly sensational holiday special called, "Twice Upon A Time." Then the mantle will be passed to Jodie Whittaker, the Thirteenth Doctor.

Earlier this year, we sat down with Capaldi and his cast members Pearl Mackie, Michelle Gomez and Matt Lucas to discuss the era of the Twelfth Doctor, and what it's like to say goodbye.

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Capaldi made it clear it was his decision to leave, which might seem surprising considering how much he's clearly relished every moment in the role. He graciously explained, "I want to leave while it's still doing good stuff. I think the idea of another year--I was worried that I wouldn't be able to deliver my best work. Because the schedule is very punishing." He chuckled, trying to imagine keeping up with all the running and the complicated dialogue and plot twists for one more go around the sun, adding with a warm smile, "I thought, 'Just go now.'"

Still, Capaldi admits his final day on set was heavy with emotion. "Of course it's sad, because it's a great role and it's a great team. But it's right, too."

Mackie backed his choice, telling us, "I respect Peter very much. He just said, 'It's my time to go.' And you can't really argue with that, can you?

"I've just been lucky that I got to work with him," she continued. "I think he's an incredible actor, a brilliant Doctor, and such a nice guy. So I just feel really lucky to explore the dynamic of the Doctor and the Companion with Peter. It's great."

In a paired interview, Lucas and Gomez recounted their last days shooting the season finale, "The Doctor Falls." Lucas said, "I focused on that it might be the last time that the three of us do a scene together," meaning himself, Capaldi and Mackie. "I felt a mixture of sadness that it might be a while before the three of us were on camera together again, but also happiness that we had the chance to work together and create something, and get to know each other.

"By the time we were shooting the final episode we were getting very close to the transmission of the series," Lucas recalled, "Previews were starting to appear. It suddenly felt real. And I felt really humbled and grateful."

Michelle Gomez as Missy on Doctor Who

For her part, Gomez described her last day on the finale shoot as "Very very sad," adding, "To start on something with Peter on the first day--and with (director) Rachel Talalay as well--and then three years later to be coming to the end of something, and to witness the journey we'd been on, and how excited we were when we first started? We would almost puke every time we walked into the TARDIS because like omigod!

"And suddenly it's like, 'Yeah, you're going to be filming with all the Daleks this afternoon,'" she said. "I'm like, 'What do you mean 'all the Daleks?!'' And you go in and there's just this field of every single Dalek that's been known to man! And you're just kind of constantly shaking and being excited about it all. And then, having to act!

"So that's kind of where we were from the beginning and then we kind of gradually grew into it," Gomez reminisced. "There was a real comfort there, a real trust there, and a real relationship because I'd known of Peter. We both come from Glasgow, and Peter was from the art school and I was the drama school. I'm a little bit younger than him, and I'd seen him cutting about Glasgow, like the coolest guy ever. And then like he's the Doctor and I'm the Master! It's like, what the fuzz is that all about!? It was that. I was enjoying that journey."

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Though Mackie was new to the world of Doctor Who--both as a cast member and a fan--she described the experience of discovering it as "incredible," adding, "I've been so lucky this series to work with people that I have watched on TV throughout my life, Peter, Matt, John (Slimm), Michelle. They're all people that I've been a fan of for a really long time. So I feel it's such a privilege to be able to be on the same screen as them, essentially. So, I'd say the series as a whole is a standout for me."

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Asked to pick a favorite moment, Lucas chose a brief bit of "Oxygen," explaining, "I had to run around the corner and dive into the scene and go, 'There's a rescue ship on the way.' (It was special) just because I never get to do things like that. So it was just hilarious. It's a serious line. But for me it was like being a kid, because I was pretending to be in Doctor Who, but actually being in it!"

For Gomez it was a moment making season eight's "Dark Water." "Oh, my God!" she exclaimed. "Walking through the doors of St. Paul's Cathedral and then them telling us that the only people who've walked through the doors of St. Paul's Cathedral is the queen and the mayor, Peter Capaldi, and Michelle Gomez! That was PHE-NOM-E-NAL.

"That was my massive pinch-me moment," she added with a laugh, "Standing at the doors of of St. Paul's Cathedral, like, 'You can't come in! You have to go around the side!' Just putting one step in and out, doing the hokey-pokey."

Mackie couldn't settle on one moment, recalling instead her favorite episodes. "In episode three ("Thin Ice"), when (Bill) really challenges the Doctor," she said, "And in five ("Oxygen") with that whole situation and potential death. That kind of vibe. And episode eight ("The Lie of the Land") I loved as well, with that whole 1984 vibe, and shooting the Doctor. I was kind of expecting people to be like, 'Oh my god! I can't believe she's done that' a little bit more. But people seemed to empathize, which is nice."

Photo Credit: © Simon Ridgway / BBC / BBC AMERICA

Capaldi couldn't choose, either, looking genuinely distressed when asked which of his eps was his favorite. "I'm very fond of them all really," he said. "Because it's my time as Doctor Who. Every single one of them has something special to me.

"There are some that are more special than others," he allowed, "I really like "Listen," I think "Listen" is the one where we saw something about the Doctor that I played that was very particular to him. Which was a kind of compulsive obsession about interesting things. And also, a deep love of humanity, which may not have appeared on the surface.

"But who cares about the surface?" he chuckled, "It's what people are like--what beings are like--underneath it all that counts. I like that. "Heaven Sent" was obviously an extraordinary experience to do that. And I like the ones we've done recently. The Christmas special and the finale I think were very special."

For our final question, we asked Capaldi if he snatched a souvenir from the TARDIS, perhaps a sonic screwdriver? "No. I don't feel the need to have any memento," he said. "I got to be Doctor Who."