Michael Sheen isn’t one to mince his words. Even before Boris Johnson finds himself at the centre of the Christmas party scandal, the Welsh star of Frost/Nixon has our PM in his crosshairs. “He’s the absolute worst of what politics can be,” says the 52-year-old, his voice rich and lilting. “A man who doesn’t seem to care or believe in anything other than his own advancement, and, as a result of immense privilege, has been able to get to the most powerful position in the country and then doesn’t use it to make people’s lives better. Everything is a game to him.” Sheen stops, reloads. “He seems to have no personal ethics, morals, beliefs, value system. So I will be immensely happy to see the back of him, not just from being prime minister but out of the political arena altogether. I hope he goes off and finds a job that has no influence whatsoever on anything in our cultural, social or economic life.” Deep breath. Exhale. “And good luck to him with that.”

Disconcertingly, Sheen delivers this diatribe while sporting a shock of white blond hair, like a vertiginously quiffed version of the Boris bouffant. Unlike Johnson, though, he doesn’t seem to be spouting hot air. The actor, after all, sold his houses to bankroll the 2019 Homeless World Cup. The following year he revealed that he’d handed back the OBE he was awarded in 2009 for fear of being, in his words, a hypocrite. And earlier this month, he declared himself a “not-for-profit actor”, pledging to use future earnings to fund social projects. He’s a walk-walker in a crowd of talk-talkers. That there is any hair similarity today – Sheen’s mane is ordinarily dark, scraggly and flecked with grey – is down to him currently filming a second series of Good Omens, Amazon’s devilishly entertaining adaptation of Terry Pratchett and Neil Gaiman’s apocalyptic novel.

Of course, transformation is Sheen’s usual milieu – a bald cap here, some peroxide mutton chops there – but his talents extend way beyond artifice. British screenwriter and playwright Peter Morgan called him “the most technically accomplished actor we have”. Certainly, he has an uncanny gift for mimicry – whether playing an obsequious, neatly coiffed Tony Blair (in The Deal, The Queen and The Special Relationship), a smarmy, roguish David Frost (Frost/Nixon), or a bumptious, beady-eyed Brian Clough (in The Damned United).

Ever since graduating from Rada and making a name for himself on stage in Peer Gynt (1994) and Amadeus (for which he was nominated for an Olivier award in 1998), he’s been drawn to misfits and eccentrics. “I’ve always been more comfortable playing some kind of contradiction between what’s going on on the surface and what’s going on underneath,” he tells me. “Often characters who have a level of performance about them. Tony Blair was performing in all kinds of ways. Frost was performing. Clough was performing. If you were with them in real life, it might not be the most comfortable experience.”

More often than not, he continues, the leading role in a romantic drama or an action film ends up being more of a plot device than a character with any discernible depth. “My instincts are always to try and make the character more interesting in some way, and that can often be to the detriment of a lot of very popular stories,” he says. “So I think I’ve either not been offered those parts because I’ve not seemed the right fit, or I’ve turned them down.”

His latest character would definitely be classed as an oddball. In the time-travel fantasy Last Train to Christmas, airing today on Sky Cinema, Sheen plays Tony Towers, a brash Nottingham nightclub impresario with a terrible Eighties mullet and a taste for cheap champagne. On a train home for the festive season with his brother (Cary Elwes) and much younger girlfriend (Nathalie Emmanuel), Towers discovers that whenever he moves between carriages he’s suddenly transported to a different decade, where he’s allowed a peek at the diverging paths his life might have taken.

Naturally, just as he hops between decades, so the hair and costumes change – we get everything from Seventies rocker and besuited Eighties music exec to a Nineties daytime DJ in very short shorts. As is traditional with festive fare, the film is funny, heartwarming and tinged with melancholy – think of it as a wintry cocktail of A Christmas Carol, It’s a Wonderful Life and Sliding Doors.

“Tony Towers is a character full of bluff and ego and nonsense, who is forced to look at himself in a way that he never has; forced to look at choices he’s made and then tried to put right, and then made even more of a mess,” says Sheen. “I like that deconstruction of a character who you wouldn’t normally see deconstructed. He’d normally be a supporting part, a laugh and a cliché and a bit of a stereotype, but you start to see there is actually a lot more to him.”

My instincts are always to try and make the character more interesting in some way, and that can often be to the detriment of a lot of very popular stories

Last Train to Christmas is one of several Sheen productions to have come out during the pandemic. He had a first lockdown hit with Quiz, the BBC adaptation of James Graham’s play about the infamous “coughing major” cheating scandal on Who Wants To Be a Millionaire?, with Sheen note-perfect as host Chris Tarrant. This June, he reprised his role as imprisoned serial killer Dr Martin Whitly for season two of Sky One’s US procedural drama Prodigal Son. Then there were two series of Staged, the charming meta-comedy in which Sheen and his mate David Tennant squabbled over Zoom as exaggerated, hyper-thespian versions of themselves, capturing the ennui and frustrations of lockdown.

In all honesty, he’s needed the work, having two years ago given “every penny that was in my bank account” to fund the Homeless World Cup in Cardiff. Eight weeks before the charity football event, recalls Sheen, there was “a bit of disaster”, leaving him with two weeks to finance it personally. “The advice was to walk away, but I just felt I couldn’t let that happen, so I had to try and convince everyone to stay on board.” He sold his properties in the US and the UK to raise the cash. “It was a lot of stress,” says Sheen, whose partner, the Swedish actor Anna Lundberg, was pregnant at the time.

But the tournament was an unqualified success, he beams. “One of the things it taught me was that I could risk a lot more of my own finances on funding things that matter to me. I realised I was making myself into a social enterprise. I was becoming a sort of non-profit actor. All the profits I made, I would pump back into the things that I cared about and that mattered to me. So, in a way, the disaster that was averted actually gave me the confidence to go a bit further.”

This is exactly the kind of altruism that gets you an OBE, I tell him. Famously, Sheen was awarded one in 2009 for services to drama, but returned it after he was invited to give the 2017 Raymond Williams Memorial Lecture in honour of the late Welsh socialist. After delving into the history of English imperialism in Wales, he says he realised that if he was “to deliver that lecture while holding an OBE, that would make me very hypocritical. So I had to either not do the lecture or give the OBE back.” Although receiving it “felt like being capped for my country, which, as someone who wanted to be a footballer at one point in life, was very special to me”, returning it was something he could live with more easily.

If Sheen the actor is driven and chameleonic, Sheen the person is laidback, with an air of rugged self-belief. Growing up in Port Talbot, the hometown of both Anthony Hopkins and Richard Burton, Sheen has showbiz lineage: his great-great-grandmother was an elephant-trainer and lion-tamer, while his father tried his hand as a Jack Nicholson impersonator. Initially, though, football was the dream, and he was offered a trial by Arsenal after being spotted by the father of ex-Gunners’ centre-back Tony Adams during a family holiday at Pontins on the Isle of Wight. “I was at my peak at 12,” says Sheen, who, in his words, “skinned” a young Adams that day. “I objectively have proof that I was really good because I often played on the Arsenal youth team.”

By 14, he’d fallen for acting. “It was one of the happiest times of my life,” he says. “It was a magic balance to have, to be learning about something new and at the same time realising you have an aptitude for it and a passion for it. It coincided with me being part of the [National] Youth Theatre. I would live in this old building with about 40 other young people who loved doing it at the same time. And a lot of them were older than me, so I remember a 21-year-old Russell T Davies was just finishing his time as a student as I was sort of starting it. The possibilities for a life were just opening up for me. My heart would be broken for the first time the following year. So I still hadn’t had that heartbreak yet. I had no responsibilities, I had nothing, I didn’t have to pay bills, I didn’t have to do anything that I didn’t really want to.”

Fast-forward to the mid-Nineties and the bills had started to arrive, but that feeling of “excitement and magic” he got from acting remained. He was cast in a touring production of Chekhov’s The Seagull, which is how he met Kate Beckinsale, with whom he had a daughter, Lily, in 1999. By the early Noughties, they’d relocated to Hollywood and the roles would follow (from Underworld and Blood Diamond to Midnight in Paris and the TV drama Masters of Sex). Sheen and Beckinsale separated in 2003, but not before he’d made headlines for punching her co-star Jeremy Northam because he thought he was “being disrespectful” to Beckinsale on the set of The Golden Bowl.

Sheen went on to date fellow actors Rachel McAdams and Sarah Silverman, but is now settled back in south Wales with Lundberg and their one-year-old daughter, Lyra. Life in lockdown has been good, he says. “There were no distractions. We got to enjoy this very special period of time with this little creature that had come into our lives, and be able to go out in the garden in nice weather with her and just focus on her.”

Sheen is sounding far too moony-eyed for his own good, so I can’t resist getting him fired up just one more time before we leave. I ask if he ever worries about cancel culture, a question prompted by the fact that one Twitter user announced her intention to cancel him for “framing” the Welsh as an “abused” people. “[Talking about it] is just a waste of time,” he responds. “That’s all you read about now. For every column that’s about cancel culture, there’s one fewer for real dangers and unfairnesses. The idea that being aware of social issues and aware of the injustices done to certain people, both historically and in the present, the idea that that gets labelled as woke and then is used as a pejorative, it just makes it so easy for people on the right, doesn’t it?” He gathers his thoughts. “It’s like microwave dinners as politics.”

‘Last Train to Christmas’ premieres on 18 December on Sky Cinema and Now