“I think one of the best things about Doctor Who is you never really truly say goodbye to it,” Pearl Mackie tells us, a few weeks before she’s due to appear in one of the biggest farewell episodes the BBC sci-fi series has ever produced.

“It sort of lives on forever. I won’t always be the companion, but I will always have been a companion to the Doctor at a certain time. No-one else is gonna be Bill Potts, ever!”

The same can’t be said for her Time Lord co-star. Our conversation is taking place mere weeks before Christmas Day audiences will see current Doctor Peter Capaldi regenerate into the character’s first female incarnation, played by Jodie Whittaker.

“I think it’s fantastic that they’re having a woman as Doctor Who, and I think it’s fantastic that it’s Jodie, I think she’s brilliant,” Mackie says. “Yeah, I think it’s gonna be a great new energy and dynamic for the show.”

But in all the excitement over Whittaker’s arrival and Capaldi’s exit, it feels like Mackie’s own farewell is being a little overshadowed. Normally the official departure of a series companion would inspire a huge amount of attention and coverage from fans and media – the possibility of  Jenna Coleman’s exit was discussed and speculated upon for years before it actually happened – but this year, nearly all the coverage has (understandably) focused on the slightly more momentous fact of Whittaker’s debut or, failing that, the last episode of a long-serving Doctor.

In a way, though, Mackie’s Doctor Who tenure has always been marked by similar upstagings. The 30-year-old actress was cast in the sci-fi series in 2016 at an odd point in its life, a few months after showrunner Steven Moffat had announced he was leaving the programme and handing over operations to Broadchurch creator Chris Chibnall.

The last time such a handover was attempted on the modern series (between Russell T Davies and Moffat), the entire cast changed too – so many viewers assumed Mackie was already leaving a year before she’d even started. Throw in lead actor Peter Capaldi’s announcement of his own departure back in January, months before her first episode had aired, and Mackie’s days on the series always seemed numbered.

“I mean I think Bill and Jodie’s Doctor would have had great adventures, but I feel very lucky to have done one series of Doctor Who,” she says when I ask if she had any wish to stick around. “I don’t want to be greedy, y’know?”

It was an inauspicious start, not helped by an introductory video for her character Bill that some fans found annoying (writer Steven Moffat later described it as a “slightly caricatured” version of the final version of Bill) – but then when her first official appearance actually aired in April, Mackie confounded her critics.

Her performance as Bill was near-universally praised, with some calling her the best companion since Billie Piper’s beloved Rose Tyler, and as the series progressed the mood didn’t dampen.

“I think Bill and the Twelfth Doctor’s partnership, it seemed to be so well-received,” Mackie says now. “It was lovely watching it onscreen – it was lovely filming it – but it was really lovely to feel their dynamic and sort of watch them together.

She adds: “And you know, what’s great about the Christmas special is you get to see them together again! One last time.

“It’s so sparky. So… real. It’s great, and I think people will really like how they interact in the Christmas special.”

Exactly how Bill returns this Christmas remains a closely-guarded secret – Mackie will only say “there’s a bit of a mystery” about how she re-encounters the Doctor after being reborn as an immortal creature at the end of the last series – but she’s happy to talk more generally about festive episode Twice Upon a Time, which also sees the Doctor encounter his former self, David Bradley playing original star William Hartnell’s First Doctor.

“David is wonderful, he’s a brilliant actor, and his First Doctor is incredible,” she says. “It’s uncanny!

“I think the interactions between him and Bill are quite interesting, and potentially not what the First Doctor would have experienced before.”

Of Bill herself, she adds: “What kind of journey does Bill go through in her last adventure? Well it’s a very exciting one, and it’s a very snowy one! It’s Christmassy, which is very fun and there being two Doctors, and meeting the Doctor at the point of him not wanting to regenerate is quite exciting.

“And you know, seeing his dynamic with the First Doctor is great. When do you get to see Two Doctors together? That’s fantastic. And when, as a companion, do you get to experience two incarnations of the same person that you know?

“It’s a wonderful kind of finale, for the Twelfth Doctor and for Steven [Moffat] as a[departing] showrunner.”

And post-Who the actor isn’t resting on her laurels, soon to return to the theatre (her main credit prior to Doctor Who was the stage production of The Curious Incident of the Dog in the Night-time) for a starry production of Harold Pinter’s The Birthday Party alongside Stephen Mangan, Toby Jones and Zoe Wanamaker (in typical British acting tradition, two of her co-stars are also Who veterans).

“I am going back on stage, yes!” she says of the new project, which runs from January until April at the Pinter Theatre.

“It’s really nice, and we actually had our first day of rehearsals for the Birthday Party yesterday. It’s a brilliant play. Pinter’s fantastic, I think he’s possibly one of the best writers that we’ve produced.

“If I’m able to continue working onstage and in front of a camera, and behind a microphone, for the rest of my career, then lucky me.”

Still, no matter what else she works on going forward, Mackie is certain that Doctor Who will always hold a place in her heart.

“One minute you’re on a flipping hill in the middle of the Welsh countryside, shivering, huddled inside the Tardis for warmth, and the next you’re in the studio, on a wire being flown horizontally,” she says.

“And there’s a wonderful kind of familial feeling about Doctor Who. Meeting previous Doctors and previous companions, while they’ve done many many things since being on Doctor Who, they still talk about it and they still feel like a part of it. And even though you’ve never met you have that thing in common, and you can talk about it.”

Now, she’s just waiting for the day her casting in the series starts to feel real…

“I’m still waiting for that!” she laughs. “Getting the job and not being able to tell anyone, and then going and filming the little trailer in secret, and then going and doing it, and being kind of thrust into this amazing juggernaut of all of space of time – it’s so big, it doesn’t ever really seem real.

“I don’t know when it’ll feel real. I think maybe watching the Christmas special.”

She laughs again.

“It still feels like a dream – I’ve said that a couple of times before I think. I haven’t woken up yet. Nice dream though!”

For more exclusive Doctor Who Christmas content, including behind-the-scenes pictures of Peter Capaldi’s last day on set, check out the legendary Radio Times Christmas issue, on sale in certain areas now and nationwide from Tuesday 12th December