When "Doctor Who" returns to screens this weekend, it's a new beginning as Pearl Mackie joins the cast. And it's the beginning of the end for star Peter Capaldi and showrunner Steven Moffat.

I caught up with Mackie, Capaldi and Moffat at the London premiere of the season 10 opener, "The Pilot". Speaking to them brought an odd mix of excitement for new adventures in the Tardis and sadness at the departure of the 12th Doctor.

"There just comes a time when it's time to go," said Capaldi, who became the Doctor in 2013. Though the production team wanted him to continue, the actor said he found it "increasingly difficult to find new ways to do it...If I was reaching the point where I wasn't allowed to try things out anymore, well, that's not really what I want to be doing."

Though Capaldi isn't leaving until the Christmas special, season 10's trailer shows a glimpse of the regeneration scene in which the Doctor transforms into another person. Capaldi hints the regeneration itself will be "slightly different from what you've seen before."

Capaldi didn't make up his mind about his future until about halfway through production of the new season, which is still being filmed. "I just kept avoiding the issue," he admitted. "I didn't want to have to make the decision. But it reaches a point where they have to know, because they have to find somebody else to do it."

The actor swears he has no idea who that somebody else will be. "I'm sure whoever it is will have a great time," he said. "I hope they have as much fun as I've had."

Writer and producer Moffat is also bidding farewell to the show at the end of the season after eight years in charge. Having tackled iconic characters Sherlock Holmes and the Doctor, the writer is ready to come up with a brand-new story. "I'm tired of writing other people's characters. I would quite like to write something completely different," he said, before adding with a rueful smile, "...and fail miserably and wonder why I left."

Incoming companion Pearl Mackie -- "Doctor Who" is her first major role -- says joining the show was like being "welcomed into a really weird, dysfunctional, wonderful family". The Doctor's previous companion, Jenna Coleman, who played Clara Oswald, sent Mackie flowers and gave advice on practising her "superhero run". And some practical advice too -- "mainly where to get nice food delivered in Cardiff [where the show is filmed], which is actually surprisingly useful," Mackie said with a laugh.

"It has to come across the border, dropped in by helicopter," joked Moffat.

Mackie also got some advice from Doctors past and present. "I met Matt [Smith] in Europe when we were at Comic-Con. He had lots of advice and I can't remember any of it."

"Can't you, really?" asks Moffat slyly, suggesting the meeting was quite a night out.

Mackie's character, Bill Potts, brings fresh eyes to the show, and so does the actor. Though she watched some Star Trek, she wasn't a fan of "Who". "It wasn't on when I was a kid," she said.

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Mackie got a glimpse of the madness that surrounds a high-profile show when she casually mentioned Bill's sexuality in an interview, sparking a media frenzy the "Who" team say took it by surprise. "Apparently there's an awful lot of journalists who haven't met anyone gay before," Moffat said. "They must find walking around Soho exhausting."

Moffat insists he had no intention of making a statement. "When I was pitching the character of Bill Potts to the BBC I forgot to mention she was gay. That's how far down the list it is," he said, adding, "We're not expecting a pat on the back for this -- this is the bare minimum of representation people should have and it's been a long time coming."

Mackie describes Bill as "real", "relatable", and "very human". Her role as the Doctor's companion is to ask questions, both to help the audience understand what's going on and to challenge the mysterious Time Lord. "She asks the questions he hates to be asked," Mackie explained. "Frequently it has never been asked before, which has been great fun. 'Why don't you use your Tardis for deliveries?' 'Why do you need a tool to get through doors when you've already got a machine that can just get there?' She's just really sort of lively and curious and amused and enchanted by this strange man."

Moffat points out that the show's very title is a question. "The essential story of 'Doctor Who' is, Doctor Who? Who is he?" He believes the answer depends on who's asking. "Rose would probably say Doctor Who is the man I love. Amy would say that the Doctor is my second favourite man."

Of course, some questions may never be answered. "I think we know as much as we're going to about the doctor," Moffat said. "There's stuff he's never going to tell us."

Capaldi is a fan of the Doctor's essential mystery. "He's not human, so I think it's wrong to conceive of him in human terms. He presents a human face so that we can understand him, but I don't think he's really understandable."

As he prepares to pass the mantle, Capaldi believes the show is fiercely relevant because it's founded on principles of "kindness and intelligence and bravery." He believes the show "is a good thing in a time when the world is struggling to recognise values that are lasting and pure."

The actor singles out the episode "The Zygon Inversion" as an example of the show's values. Depicting extremist factions on both sides of an alien invasion, the story ends with Capaldi's Doctor delivering a heartfelt speech about the pain of conflict. It was broadcast in November 2015, the week before a terror attack in Paris killed 130 people.

He remembers that the classic series tackled issues of the day through a sci-fi lens, like the 1972 serial "The Curse of Peladon" that riffed on Britain's entry into the European Economic Community. Capaldi would like to see the show "reflect the world more...trying to do it in a way that children could understand."

"Doctor Who has always been the show that's always said the right thing at the right time," Capaldi said. "At that moment it was exactly what the world needed and exactly what the fans of the show needed to hear."

Today, "Doctor Who" is bigger than ever, with episodes screened live around the world to hordes of fans. But Capaldi smiles as he remembers his own humble introduction to Doctor Who. "It will still always be that little teatime show that starts on BBC One after Grandstand," he said. "That's the trick -- I love the show, so when we're in a tight corner while we're filming it, I draw upon my memories as a child to try and create the atmosphere that I remember."

"Doctor Who" returns on 15 April on BBC1 in the UK and BBC America in the US. You can also check out a double bill of the first episode with the premiere episode of "Who" spin-off "Class", featuring a cameo from Peter Capaldi, in US movie theatres on 16 and 17 April.

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