The 66-year-old actor, who's been called the world's most terrifying actor, told Sky News he had the realisation he'd "gone sinister" while recording a voiceover for Anchor Butter.

About to reprise the role of serial killer Gideon Shepherd in series two of psychological thriller The Devil's Hour, it's a quality the Scottish star is making the most of.

Capaldi explains: "I don't quite know how that happened… When I was a young actor, I always played sort of slightly geeky and pleasant, easy-going sort of people. And then somewhere - probably around Malcolm Tucker time - I began to change."

He played tyrannical spin doctor Malcolm Tucker in BBC sitcom The Thick Of It for seven years, as well as reprising the role in the 2009 movie version.

Widely considered to have been based on spin doctor Alastair Campbell, who was aide to Tony Blair during his Labour leadership, Capaldi's portrayal was ripe with ruthlessness, ranting and extreme profanity.

The role was a fan favourite, turning Capaldi into a household name.

Capaldi goes on: "The key moment for me was doing a voiceover for Anchor butter, which [culminated in the line] 'Anchor, tastes like home'.

"Then one day, on what I didn't realise was my last voiceover, they said, 'Could you sound a little less sinister?'

"I thought, 'Oh no, I've gone sinister, and I don't know how that's happened'. But then I thought, 'Well, if people want to buy it, that's fine'."

'A time travelling timey-wimey kind of character'

In 2013, the year after the final series of The Thick Of It, Capaldi became the Twelfth Doctor in sci-fi classic Doctor Who.

A fan of the show as a child, he was widely praised for reinventing the role, with a heavy dose of grumpiness at the outset, transitioning to kindness by the end of his four-year tenure.

He says he wasn't worried about time-travel typecasting for his role in The Devil's Hour, which also sees him bounce back and forward in time.

Executive produced by Steven Moffat - head writer of Doctor Who during Capaldi's stint - along with Moffat's wife, acclaimed TV producer Sue Virtue, the actor credits them with being "very gifted" programme makers.

Capaldi explains: "I think they probably felt with a sort of time travelling timey-wimey kind of character, I might fit that. And that's fine.

"I guess I'm able again for some reason to kind of plug into a kind of cosmic vibe, so that was useful for Gideon, and I'm quite happy to do that."

'I've got terrifying and cosmic things in my toolbox'

With early gigs in theatre, before TV and film roles followed, Capaldi looks back to his youthful knock backs as his building blocks for latter-day success.

He admits: "The thing that stops you getting work when you're young, is often the thing or things that will bring you work when you're older.

"It's your individuality. It's the things that are odd, the things that are different. And if I've got terrifying and cosmic things in my toolbox that are mine then that's good, they get me work."

Capaldi labels himself a "lucky" actor, explaining: "Life is unpredictable.

"You can be tootling along thinking things are going fine, and then some difficult challenge can suddenly floor you.

"By the same token, the stars can align, and your life can be transformed in a very positive way, through nothing that you've done.

"Certainly, in my experience, things have happened to me that were pure luck - I just happened to be in the right place at the right time or available."

Brutal and violent - but not gratuitous

Other recent roles include a morally grey police chief in Criminal Record, supervillain The Thinker in Suicide Squad and tragic war poet Siegfried Sassoon in biography Benediction.

He's also been announced as one of the stars of the next season of Black Mirror, which will air next year, and while details of his part are yet to be confirmed, Charlie Brooker's dystopian anthology is unlikely to be full of rainbows and sunshine.

But it's his latest role in The Devil's Hour, playing a criminal mastermind who "remembers the future", that is the darkest so far.

The storyline involves multiple murders - including those of children - terrorism and domestic abuse.

But while the crimes are grisly and brutal, they're not portrayed gratuitously on screen.

'I've got a grandpa thing going on'

Like his co-star Jessica Raine, Capaldi admits his tolerance of depictions of violence around children in TV and film hit a wall when he became a parent.

Father to one daughter, now 30 with two children of her own, Capaldi says: "I feel that very powerfully. But that's the business we're in.

"I think we're all very warm and concerned and protective of our children. But at the same time, they love Grimms' Fairy Tales.

He goes on: "I was just about to say maybe everything should be Disneyfied. But then, you know, all of those old fairy tales are full of the most terrible violence and horrors.

"There's a kind of fine line between trying to keep children safe and keeping them aware of the world. It's not all Disney."

Then, after a beat, Capaldi adds: "Although I am available if Disney are watching... to bring my cosmic and terrifying but cheerful toolbox."

After five decades of success in the notoriously fickle world of showbiz, Capaldi's chameleon-like nature continues to bring him work.

And always looking to the future, he adds: "I've got a grandpa thing going on now, that might be quite useful."

Season 2 of The Devil's Hour is streaming on Prime Video from Friday 18 October.

Season 3 of The Devil's Hour, which has already been filmed, will air in 2025.