“I don’t think it’s too scary for a three-year-old, is it?” David Bradley asks, worried that having watched his appearance in the upcoming Doctor Who Christmas special, his granddaughter might spend the rest of the day hiding behind the sofa.

He pauses, concerned. “It’s the Cybermen… I mean, they freak me out!”

Bradley’s own Doctor Who journey began at a more mature age, when as a 21-year-old he watched William Hartnell emerge from the Tardis for the first time back in 1963.

“You’d have your tea, watch Doctor Who and then go out on the town when you were in your early 20s,” the now 75-year-old actor recalls. “I’d never have dreamt of being in it because at the time I was an engineer.”

“So I would have had to seriously question the sanity of anybody who suggested that it might be a possibility.”

Of course, it was still a long road before Bradley took his first steps into the Tardis. After leaving engineering for drama school in 1966 (coincidentally the same year William Hartnell left Doctor Who due to ill health), Bradley went on to rack up an impressive theatre CV, winning an Olivier Award for a production of King Lear in 1991 and also taking on roles in TV and film.

It was in later life, though, when Bradley became more well-known after being cast as malicious school caretaker Argus Filch in the Harry Potter film series and evil Lord Walder Frey in Game of Thrones – villainous roles that led to his first Doctor Who appearance in 2012, where he played a vicious space trader called Solomon who clashed with Matt Smith’s Doctor in an episode penned by Chris Chibnall (who takes the reins of Doctor Who entirely next year).

“That was going to be my once-in-a-lifetime Doctor Who experience,” Bradley says of Dinosaurs on a Spaceship (above), which at the time he counted as a career highlight. “And then what happens?”

Well, what happened was that while watching the Queen’s Diamond Jubilee flotilla in 2012 Bradley found himself tapped on the shoulder by Mark Gatiss, who was working on a drama about Doctor Who’s founding for the series’ 50^th anniversary and needed an actor to play the late Hartnell himself.

That story became 2013’s An Adventure in Space and Time, with Bradley’s performance as the sometimes irascible and increasingly frail Hartnell attracting praise – and so when series writer Steven Moffat decided the perfect goodbye to Peter Capaldi’s incumbent Doctor would be to bring back the very first version of the Time Lord, Bradley got the call once more.

“It’s part impersonation, part capturing some of those mannerisms — but not just a direct mimicry,” says Bradley of playing Hartnell’s Doctor, rather than the man himself. “I wanted to make it my own, while honouring his performance.”

The end result sees Bradley’s slightly more serious Time Lord clashing with Capaldi’s ragged, punk-like Doctor, while the current Doctor’s more PC attitude puts him at odds with some of Bradley’s Doctor’s 1960s attitudes.

“There’s a lot of fun to come out of that dynamic, between the two of them,” Bradley says.

“Hopefully, it’s kept a kind of lightness of touch and has a comic energy, although there are sometimes some dark moments in it.”

Of course, the elephant in the room for this year’s Christmas Special is that many fans will be most looking forward to the episode’s final moments, when Capaldi’s Doctor regenerates into Jodie Whittaker’s new female Time Lord — and Bradley, who worked with Whittaker on ITV’s Broadchurch in 2013 (alongside Chris Chibnall, who cast them both) is as excited as anybody.

“When I heard it was Jodie I thought, ‘Well, that’s perfect’ — because she’s got the range and she’s funny,” Bradley says.

“They just need to keep that sense of fun and not forget the comic energy – no matter how dark the situation is in the story. And just enjoy it, and keep that sense of fun. She’s got that, all she needs to do is tap into it, and I’m sure she’ll do the biz.”

For now, though, Bradley gets to enjoy being the Doctor himself – which also means he has the ideal present for his two grandkids.

“I’m looking forward to receiving my figurine, so I can officially say I’m Doctor Who,” he says. “Part of a pantheon. If my grandkids want one I’ll treat them.

“Unless it’s a bit too early to be thrusting replicas of myself at them…”

This interview appeared in a condensed form in the 9th-15th December edition of the Radio Times magazine

Doctor Who: Twice Upon a Time airs on BBC1 on Christmas Day (Monday 25th December) at 5:30pm