The moment Patrick Stewart arrives for our interview, he's a little agitated. Immediately, he relocates us from the rather chilly hotel bar to the much-warmer parlour. He sits down, orders a pot of tea and lets out a sigh. "Last Wednesday, I stupidly dropped my iPhone in the bath, and my life has sort of spiralled almost out of control." While I sympathise with the situation, I must confess there's something faintly amusing about the man who played Jean-Luc Picard, the captain of the Starship Enterprise in Star Trek: The Next Generation, being stumped by a waterlogged phone. "The only still centre of my life is Macbeth," he adds. "To go back to doing this bloody, crazed, insane mass-murderer is a huge relief after trying to get my cell phone replaced."

Stewart is currently filming a version of Shakespeare's Scottish play, which is why he is holed up in a hotel, just a few miles outside of Worksop, on the edge of Sherwood Forest. Fortunately, he arrives sans costume, instead dressed anonymously in jeans and a grey jumper. Yet even if he'd worn his blood-spattered robes, your eye would still be first drawn to what one interviewer described as "the most famous cranium in the world". Bald since he was 19 – alopecia runs in his family – this accident of fate has since defined him. Now 69, there are wrinkles, of course, especially around his eyes, and a caterpillar of a grey moustache, but it's this glorious, shapely, shiny head that you find yourself studying.

It has lent Stewart an almost alien-like quality that made him perfect casting for Star Trek, but it's been equally useful in portraying people of power – his Lenin in the 1970s mini-series Fall of Eagles springs to mind. As for Macbeth, he first played the king back in 2007, at the Chichester Theatre Festival, before it transferred to the West End, winning Stewart a Best Actor gong at the Evening Standard Theatre Awards. The play then went to New York, where Stewart was nominated for a Tony. Now getting the small-screen treatment, with the original cast and director reunited, it's little wonder Stewart calls the project a "no-brainer".

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Before that, however, comes Hamlet – a similarly filmed-on-location version that reunites the personnel behind the Royal Shakespeare Company's 2008 production, which featured Doctor Who star David Tennant as the titular Prince of Denmark. With Stewart reprising his dual roles – as Claudius and the ghost of Hamlet's father – television audiences will get a chance to see them recreate their stage magic when it's shown on BBC2 this Christmas. With "a very brilliant and popular actor" like Tennant – Stewart is rubbing his hands with glee – "it's possible you might end up with [viewing] numbers that are unprecedented for any production of Hamlet in the history of the world. It's possible. Wouldn't that be great?"

As he talks, Stewart's eyes

light up, energised by all this talk of Shakespeare and the stage. "The past five years have been the best years of my life and career," he tells me, something he attributes to his return to England and the theatre in 2003. By this point, he had been living in Los Angeles for nigh-on 17 years. After arriving to play Picard in 1987 for a show many advised him would be cancelled after one year, he wound up completing seven seasons and four films. Added to this, he landed the plum role of Professor Charles Xavier in Bryan Singer's comic-book blockbuster X-Men and its two sequels – and Stewart was a star. He was "living a very pleasant lifestyle", he says, and then he got a call to return to the London stage to appear in a production of Ibsen's The Master Builder.

"I took it up, not realising then how that experience would bring to the surface my discontent with the work I was doing in Hollywood and my unease about the prospects that lay ahead of me," he says. So he wasn't satisfied with appearing in the X-Men franchise? "No, no!" he cries, sounding horrified. He had even stopped buying English newspapers, for fear of seeing advertisements for plays he felt a yearning to be in. "I had come to the point when I realised it was unlikely that my film career was going to move beyond a certain level of role. And I was – because I had graphic instances of it – handicapped by the success of Star Trek. A director would say, 'I don't want Jean-Luc Picard in my movie' – and this was compounded by X-Men as well."

At the time, Stewart's second marriage – to Wendy Neuss, one of the producers of Star Trek – had come to an end after just three years. But long before this he had dreamed of returning to England, and in particular to the countryside of Gloucestershire, Warwickshire and Oxfordshire, which he fell in love with during the 15-odd years he worked for the Royal Shakespeare Company, pre-Picard. "My fantasy had always been that I would end up with a nice little house somewhere in the Cotswolds. And I would literally put myself to sleep at night in Los Angeles, by creating a virtual imagined home. Where would I live? What part of the Cotswolds? What sort of house would it be? And added to that, I would drive from my house to Stratford every day where I'd have some nice parts to play."

Stewart got his wish, rejoining the RSC after The Master Builder, and going on to play Mark Antony in Antony and Cleopatra and Prospero in The Tempest. Since then, he's barely stopped. "The fact is, I'm getting more out of acting than I ever have," he says, wedging one foot against the radiator. "I'm enjoying it more and I'm actually having more fun in every possible way. I think I possibly took everything too seriously for far too long." He credits his colleagues on Star Trek for helping him loosen up. "I think it's a healthy way to be. I think I came back from America a funnier and nicer person than I went." Certainly, after voicing characters in The Simpsons and Family Guy, as well as appearing on Frasier and Ricky Gervais's Extras, this can't be denied.

Stewart attributes his earnest nature to his childhood, growing up with two older brothers in the Yorkshire town of Mirfield. "I never had teenage years," he says, quietly. He was made head boy a year early – "I guess because I was seen to be more adult than anybody around me" – and left school at 15 to become a cub reporter on his local paper, The Mirfield Reporter. But he had already discovered acting – after a teacher had encouraged him – and his journalism career was short-lived: the editor gave him an ultimatum when he realised that his employee was spending all of his time at the local theatre. "I left the paper that afternoon," grins Stewart. "I packed up my typewriter and walked out."

It set Stewart on his way. He won a scholarship to the Bristol Old Vic Theatre School before joining the RSC in 1966, in his mid-twenties. Yet if anything turned Stewart into an adult before his time, it was his life at home. His father, a former regimental sergeant major in the British Army, was prone to outbursts of "repeated violence against my mother", as Stewart wrote in a recent newspaper piece. "I wrote the article for my mother, who has been dead for decades," he explains. Stewart has recently become a patron for Refuge, the organisation dedicated to providing shelter for those suffering from domestic abuse. "There are women just like my mother who can now take an initiative to protect themselves or to remove themselves from these situations, which my mother couldn't. And that makes it worthwhile."

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I wonder whether his relationship with his father improved when he finally left home. "Yes. It did," he replies, without flinching. "He was a very charming, charismatic man, and a great storyteller. But I have only recently come to understand that after I left home, the abuse continued for a long time, and I wasn't aware of it. One carries a certain amount of guilt and remorse for that, for not knowing, and not having done something about it at the time... The interesting thing is, my brother and I were talking about it this weekend. There are people we know who would've read that article and said it was a pack of lies. They would never believe that he could be responsible for those things."

As Stewart explains, in 1945, his father "was a superstar" in the Army. "He was so respected, and admired and feared, too." But then he had to return to work in a series of semi-skilled labouring jobs. Stewart says he was a "weekend alcoholic": it was "the combination of alcohol and his profound frustration as a man" that led him to violence towards his wife. Even so, it's clear Stewart feels like he hasn't told the whole story. "I wrote in an e-mail to somebody the other day that in a way what I'd like to do now is write another article, listing all the great things I got from my father: discipline, organisation, ambition, storytelling and socialism."

A committed trade unionist, Stewart's father instilled his political beliefs in his son from an early age. The actor recalls being a six-year-old boy, standing outside a polling station in Dewsbury, supporting the election of local Labour MP William Paling. "It was my first act of civil disobedience," he grins. "This policeman told me to walk along or he'd tell my father." Ever since, Stewart has been "an absolute fundamental, emotional socialist" and a lifelong Labour supporter.

How does he feel, given there's a strong chance the Conservative Party could win the next election? "I'm horrified. For one thing, I know the country will regret it, very, very quickly." What does he think of Tory leader David Cameron? "I wish I knew more about David," he sighs. "Maybe we will learn more about their policies in time to come. But I see a lack of substance and a kind of fence-sitting about issues that makes me very nervous. And I can't help feeling that good, old Conservative values will come to be paramount ... We will return to some of the worst of the Thatcher years. I just don't trust what I see or hear."

Nevertheless, he's still committed to living in Britain, despite currently dating the Brooklyn-based jazz singer Sunny Ozell, whom he met when he was performing Macbeth in New York. Not unlike his previous girlfriend, British actress Lisa Dillon, Ozell is some 37 years his junior. While the tabloids had a field day with this, Stewart maintains he just doesn't meet women of his own age. At any rate, he's evidently smitten. "We are trying to come to terms with the business of being separated by 3,000 miles of ocean."

It does mean he'll also be able to hook up with his son Daniel, the product – along with his daughter Sophia – of his first marriage to Sheila Falconer, a former choreographer with the Bristol Old Vic, which came to an end in 1990 after 24 years. Now an actor, Daniel is currently in rehearsals with the New Jersey Shakespeare Festival in a production of Twelfth Night, playing Malvolio, a role Stewart himself took on back in 2007. Having also appeared in an episode of Star Trek, playing Picard's son, it seems Daniel is destined literally to follow in his father's footsteps.

Stewart admits he was "horrified" when his son announced he was going to be an actor, but there was little he could do. Plus, he felt some guilt around his relationship with his children. "I could've done better as a parent, when my kids were little," he says, referring to his time at the RSC in the 1970s before he began to win television roles on dramas like Tinker, Tailor, Soldier, Spy and I, Claudius. "I was just obsessed with my work and everything else took second place. I'm trying to correct that now wherever possible with my grandchildren [he has four]. And my own children seem to have forgiven me."

Yet Stewart is still devoted to his work. "Thank God I have the energy, and the passion has never gone away," he says. He recently finished a successful 22-week run of Samuel Beckett's Waiting for Godot, alongside Sir Ian McKellen, and took off an "unprecedented" 10 weeks.

He's already thinking ahead to next year, when he will appear in two as-yet-unannounced plays, one of which will see him reprising a role he played 25 years ago. But despite loving the new Star Trek film, the one thing Stewart won't be doing is making a return to the franchise, he insists. "The thing about Star Trek is that you're never dead, really. There's always a way of bringing somebody back to life. It would be fun. But I think we've all hung up our space suits for the last time."

For a long while, slipping on his dancing shoes seemed more likely. An avid fan of the BBC's Strictly Come Dancing – having danced with legendary Leslie Caron in 1995 film Let It Be Me – Stewart toyed with the idea of being a contestant. "I rather fancied myself taking part in that. But it's a huge commitment – and one I might've been prepared to give, but I think it's too late now."

Then he lets me in on a secret. He'd also love to be a contestant on Maestro, the BBC reality show that followed eight celebrities in their battle to conduct an orchestra. "I think it was the only time in my life that I programmed my Sky Plus thing. I never missed an episode. When it finished, I thought, 'That's the reality show I want to be in. What a thrill!'" He got the bug after being invited to conduct the University of Michigan marching band – at half time during a college football match – while over there performing Shakespeare.

Should Maestro return to the screens, Stewart says he's "pitched" himself to be in it. Something makes me think he'd do rather well.

'Hamlet' screens on BBC2 on 26 December and will be available on BBC DVD from 4 January, priced £19.99