The label 'legend' is bandied around all too willingly these days. But Sir Patrick Stewart OBE is definitely a genuine contender for such a title. A recent viral photo of the actor – beachside in Jamaica, drink in hand, flexing – was all over international media; he was celebrating a Golden Globe nomination for best actor in Blunt Talk.

Although now 75 years old, he shows no signs of slowing down. It helps that he has the physique of a man 20 years younger. "I think my peasant genes sort of contributed to this – and the good fortune of moving to California 30 years ago where there was an obsession with health, keeping fit, and taking care of yourself," explains Stewart.

Rewind to 1966, and Stewart's career began as a Shakespearean actor. And not just any Shakespearean actor – a thoroughbred, from the most prestigious Shakespearean theatre group in the world, the Royal Shakespeare Company. "I spent 15 years on the stage at the beginning – a lot of it with the Royal Shakespeare Company; it was role after role, each contrasting and different from one another," says Stewart.

When he was with the Royal Shakespeare Company, he was part of a group that undertook one-week residencies at various colleges throughout the USA and Canada. "We would hold masterclasses and give demonstrations, workshops, and acting classes. We would even lecture in English literature classes – and sometimes I would just come on my own if I had a little bit of free time. I enjoyed this work very much – I tended to do it without my agent's knowledge. Because you know, if you do something like that an agent begins to worry that maybe you're not really an actor, you're a teacher," explains Stewart. Throughout Stewart's career, his love for the craft was never derailed by critics, agents, or people with other interests and agendas.

"I kept it to myself; and one morning I received a call from my LA agent, who I had never met by the way, he was just a name on a piece of paper. He introduced himself and said – because I was then living and exclusively working in the UK – 'I have two questions for you: what the hell were you doing at UCLA last night? And why does Gene Roddenberry [creator of the original Star Trek TV series] want to meet you, because of it?.'"

It just so happened that Robert Justman had been in the audience with his wife for one of Stewart's demonstrations at UCLA. Justman at the time was one of the original producers on Star Trek: The Original Series, and happening to be looking to cast people for Star Trek: The Next Generation. "He had seen me and he has always claimed that at some point during this academic evening, he turned to his wife and said, 'We've found the captain.' And so that very morning I jumped in my car and drove up into the Hollywood Hills and had a meeting with Gene Roddenberry and Robert Justman, and another one of the producers who didn't stay with the show for long. And it was a very uncomfortable experience because I felt – except for Robert Justman – there was very little interest in my being there. When I left Roddenberry's home, I thought to myself 'Well, that's one ridiculous idea, that's never going to come fruition.' But of course, six months later it did," adds Stewart.

Star Trek producer Robert Justman saw me acting and said, 'We've found the captain'

The successful meeting at Roddenberry's home lead to an audition with the Paramount executives: "I auditioned and was – to my astonishment and confusion – offered the role of Jean-Luc Picard. That's how it all came about," explains Stewart.

He went on to play the TV role for seven years before reprising it in four feature films.

Iconic characters like Picard don't come along all that often in an actor's career – if at all. But Stewart has a knack of landing them. He's now known – including by a whole new generation – as Professor Charles Xavier in the X-Men franchise. Xavier is another strong yet benign character – like Picard, he's a hero among men (and mutants).

Yet in his latest movie, Green Room, he plays someone very different. If you expected to see him playing another good guy destined to save the world, you expected wrong. "My character – Darcy Banker – is a very, very bad guy." The surname should have given it away, right? But, in fact, Banker is a club owner who, with the help of his cronies, doesn't hold back when it comes to torturing people. In the film, a young punk rock band is in the green room of a venue waiting to perform a show. They end up witnessing an act of violence that they were not suppose to see. The band tries to intervene – not a good idea. "The band ends up at this strange, abandoned barn to perform a gig. It's a very extraordinary situation," explains Stewart. And one that becomes extraordinarily gory, too.

"We filmed in Oregon; the location was about 20 miles outside Portland, in the mountains and the forests. It was a very remote and rural environment, perfect for the subject of the movie," explains Stewart. The main venue for the film is particularly eerie: "It was all a very effective part of creating an oppressive, claustrophobic, and disturbing atmosphere." At one point in the movie, they brought pit bulls in to heighten the gore. "They were very well trained, beautiful dogs. I have some experience with pit bulls, which have this fearsome reputation of being brutal and savage, but they're not – they only are if they've been brought up that way. In fact, pit bulls can have the sweetest, gentlest natures, and they are great care dogs," adds Stewart. "I took a lot of pleasure hanging out with those dogs – even though in the film they were tearing people's throats out," he says.

Green Room is directed by Jeremy Saulnier and was first screened in the Directors' Fortnight section at the 2015 Cannes Film Festival. While still delivering enough thrills to keep horror fans happy, it's impressed the critics for its intelligence, craft and acting elan. No doubt Stewart has been the key player in this.

Patrick Stewart's most famous roles

Diversity in roles is something Stewart enjoys the most: "Only three years ago I was on Broadway doing two plays in repertory – we would perform both plays on Wednesdays and Saturdays, a matinee and an evening. People were often expressing surprise that we – Ian McKellen, Shuler Hensely, Billy Crudup and I – would be playing totally different sets of characters on the same day, as though there was some mystery to it. It's all part of what actors do," says Stewart.

Stewart's contribution to theatre and the arts has been suitably rewarded: "Being knighted by the Queen was thrilling and absolutely confusing, because it was something that I had never expected. I had already been made an Officer of the Order of the British Empire (OBE) [in 2001] – like Eddie Redmayne was last year – but I never expected it to go beyond that. The knighthood [in 2010] was astonishing and very, very satisfying, because I took it as an award for all the work that goes on in British theatre and British films, and that it was a recognition of not just myself but everyone that works in the British theatre and film industry. I'm very proud to have it – and I use it very sparingly," he says.

Stewart spends a of lot his leisure time appreciating art of a different kind: "My wife and I love going to galleries; we are collectors. And she, in particular, has a passion for medieval art. London, of course, has one of the finest collections of medieval art in the world," adds Stewart. In fact, you may bump into Stewart at the Tate Britain or Tate Modern – he is very fond of both of those galleries as "neither of them are overwhelmingly massive, and they have outstanding collections," he continues. "So we spend a lot of time looking at paintings, looking at sculptures, and so forth. And that's one of the great appeals to many wonderful European cities, out of which of course, I think London is the best. I'm having a second love affair with London these days – it has become a more beautiful, attractive, and exciting city with every year that goes by," Stewart explains.

If you happen to have seen Stewart in the red tops or on the red carpet, one thing that stands out the most is he is always immaculately dressed. He has a keen fashion sense, adapting to any environment, whether it's a lavish dinner, press event, or even at the beach, he's always in style. "Well, I have been, for a number of years, a great fan of designer Paul Smith, and I wear a lot of his clothes. In fact, I wear Paul Smith in my new comedy series Blunt Talk. The first season aired last year, and we're currently in the middle of production on our second season. It's about a journalist who has a nightly cable news program, and I wear Paul Smith when I'm in front of the cameras, shooting things to do with the show," he continues. "Another designer that I wear a lot of is John Varvatos – an American designer, of course. Certainly for casual clothes, I think he's terrific."

I'm blessed to have a team of outstanding money managers and advisors who work with me

When it comes to managing his money, he pays attention: "I grew up in a very poor blue-collar family. I think I'm accurate when I say 'very poor'; we were. We basically lived in two rooms – one bedroom and one living room – no bathroom, no toilet, and no kitchen. And I lived like that for 15 years. So I also come from a part of England, North Yorkshire, where we were taught to be very careful with money. So yes, I am. I pay attention to that, but I'm also blessed with having a team of people that work with me – money managers and advisors – who are outstanding. And I listen to them, and I take their advice."

There's plenty coming up for the actor this year, with the imminent release of Green Room, before he joins fellow X-Men star – and old pal – Sir Ian McKellen in a UK tour of Harold Pinter's classic stage play No Man's Land. Whether it's down to his upbringing, genetics, or happy attitude, expect to see much more of Sir Patrick Stewart, because he shows no signs of slowing down.

Green Room is out in cinemas on 13 May 2016.