'Charles Xavier is a very, very dangerous individual' (even if he needs medicating) says Patrick Stewart. But actors live in a world of make-believe, Stewart says. "We live in a giant nursery where we get to play very expensive games. So I don't think [age] quite applies to us in the way that it does to others," he says. "Our job is becoming other people, taking on another life." At times, Stewart says, even he sees his life as a lifetime role. "Do I feel 76? No, not for a moment," he says. "I was 40 the other day ... I can remember every detail because I was, for my 40th birthday, in a castle in County Tipperary in Ireland filming Excalibur." And, he adds with a sly smile, "I know everything that happened on that day, most of it not good." There are moments when the older, present-day Stewart doesn't seem so different from the 17-year-old, who was working in regional theatre in the UK and scored a 14-month world tour with the Old Vic company, in the ensemble of three plays headlined by the legendary actress Vivien Leigh.

Hugh Jackman and Patrick Stewart: Reunited in Logan. Credit:Getty Images "Vivien played the leading role in each one of them and I had only I think eight words to speak in all three plays added together," Stewart recalls, laughing. The tour brought him to Australia in 1961, performing in Melbourne, Sydney, Brisbane, Adelaide and Perth. "We were in Australia for six months, and I had my 21st birthday in a flat on Toorak Road, in Melbourne. And Vivien came to my 21st birthday party." Patrick Stewart as Professor Charles Xavier and Hugh Jackman as Wolverine in Logan. Credit:Fox Looking back at the 21-year-old man, Stewart says his background and what he'd learnt as a professional actor prepared him for the fact that there was no promise. "There was only possibility," he says. "And the odds were very bad for it being a successful career. I was content to just be working."

When he finished his schooling at the Bristol Old Vic Theatre School, he was one of only a few in his class who did not secure a manager. Patrick Stewart arrives at the Logan premiere in Berlin. Credit:Getty Images "I was firmly convinced that my career was over before it had begun," he says. "I went back to my parents' home. I had no money and I thought, well, I had my shot and that was it." It didn't quite turn out like that. In addition to a stellar career on the British stage, Stewart made the successful leap into Hollywood playing Star Trek's Captain Jean-Luc Picard in Star Trek: The Next Generation and the X-Men's mysterious mentor Professor Xavier. Along the way there were extraordinary roles – Sejanus in I, Claudius is one – and a chance to work with legendary directors in their own creative infancies: John Boorman, playing Leondegrance of Camelyard in Excalibur in 1981, and David Lynch, playing Gurney Halleck in Dune in 1984.

For a young actor whose first television gig as a fireman in the British soap Coronation Street in 1967, it was a dazzling time. It also taught him one powerful lesson: be prepared. "Right up to the present day, working with Jeremy Sornier on the movie Green Room, I began to feel after two or three days of work he had the cut movie already running in his head and I completely trusted that vision," Stewart says. "If he said do this, do that, go there, speak like this way, I did it. "Even if the thoughts and the ideas were wrong or inappropriate, at least I would have prepared something," Stewart adds. "And of those directors, they were always full of surprises, despite their preparedness and their concentration." Logan director James Mangold was no different, Stewart says. "James has strong attitudes towards the script, he has a vision for the script and, this is the most important bit, he also has a relationship with his actors, his director of photography, the sound department, everybody on set," he says. Equally, he adds, Mangold was open to improvisation. "A sense of, I know what I want, but surprise me," Stewart says. "I think there can't be an actor alive who doesn't respond well to that."

There were points in Logan, Stewart recalls, when Mangold said, "When we get to the end of the scene, don't stop talking, just go on playing the scene." Such moments, Stewart adds, are for an actor an "exciting and liberating experience". The film itself steps deeply into the Post Traumatic Stress Disorder (PTSD) superhero movie trope, the idea that such comic book heroes are not only not invincible, but that they must ultimately pay a human cost for their lives as super soldiers. "We have characters who have become known for being warriors, fighters for the common good and who have always been prepared to sacrifice everything for that cause," Stewart says. "Now we very quickly find that the surroundings of the X-Men world have gone; it is a different world that we're living in and Charles Xavier and Logan are struggling to survive." Complicating the situation is Charles Xavier's age and what seems to be the degeneration of his powers, at least in terms of his ability to control them. "His unpredictability has now become not a blessing on the world but a danger," says Stewart. "Charles Xavier is a very, very dangerous individual and has to be controlled. And, in this story, has in fact been medicated. Otherwise bad things are going to happen and he has done bad things. Our film only hints about what he has done."

In this here and now, the status quo is bleak. "All bets are off as far as the world of the X-Men is concerned and that's exciting," says Stewart. "And I think the fans will be excited by it too and satisfied by it. Charles' nature, his temperament, his vulnerability, just like Logan, both of them are no longer entirely able to take care of themselves." The film, which takes its name from one of the aliases adopted by Wolverine, also reunites him with Australian actor Hugh Jackman, who is playing Wolverine for what looks also to be the last time in the film franchise. "Watching Hugh's performance, that's Logan, there is the Logan that I've known for 17 years," Stewart says. "But there is now a frailty about him, a vulnerability, a sense of weariness and age, of insecurity as to how he might be able to save a situation and control it. It is, I think, the most fascinating aspect of this film." Unusually for an actor, Stewart has enjoyed several long tenures with characters: either continuously, such as his seven-year-run as Captain Jean-Luc Picard in Star Trek: The Next Generation, or periodically, as Charles Xavier in the X-Men movie franchise.

Both are fundamentally different from the more common re-occurrence in an actor's repertoire, a role such as Macbeth, which can be revisited many times but never moves past the framework of Shakespeare's text. "Certainly if we look at Jean-Luc Picard, there came a point quite early on in the seven years when we were shooting that series when I didn't have to sit in my trailer before the day's work thinking myself into the role of Jean-Luc Picard. I was him," Stewart says. Charles Xavier is a little different, he says. ""Because we had these breaks all the time. And the circumstances of all of the movies has always been a little bit different, and never as different as it is with Logan," he says. As we finish our interview, Stewart is heading off to the rest of his day, a breakneck-paced schedule of promotional interviews and appointments. Despite his preoccupation with old age, he's light on his feet, and certainly fitter than me, trailing a few decades behind. More than anyone else, he says, he is constantly amazed by his fortune. "I would have settled for what I had 50 years ago. But it turned out a little differently," he says, smiling.

Just a few weeks earlier, he says, he was invited to dinner with legendary actor Kirk Douglas and his wife Anne, to mark the Spartacus star's 100th birthday. "He had been one of my heroes as a kid. I mean as a kid I had adored his work and that was before I was old enough to go and see movies like Detective Story and Ace in the Hole," he says. "And it was one of the most invigorating and encouraging hours that I have spent in a long, long time." Loading Douglas and his wife riveted Stewart with stories, he says. "And there was a moment when I thought, well between the three of us we're like 270 years old. But there was really no sense of that at all. I think there is something that gives to actors a kind of, sort of eternal youthfulness in some respect. And it's because our job is playing games." Logan opens in cinemas on March 2.