‘Fame’, writes the American author Erica Jong, ‘simply means millions of people have the wrong idea of who you are.’ It is the day of the BAFTA screening of Alejandro González Iñárritu’s The Revenant – a brutally beautiful tale of survival and revenge set in the wilds of the 19th-century American frontier – and several rooms in London’s Claridge’s hotel are filled with print and television journalists, each with their own notion of who Leonardo DiCaprio is. Many have flown in from all over the world to be here; most haven’t touched the immaculate sandwiches on silver platters that have been laid on to ease the hushed anxiety of their wait.

There are a handful of certainties about ‘Leo’ – as the world, like his friends, tends to call him. He is 41 years old, a Scorpio and the most bankable male movie star in Hollywood. Nobody can touch him: neither Tom Hanks nor Tom Cruise, not Brad Pitt, Johnny Depp or Matt Damon. If DiCaprio says he wants to be in a film, it not only gets made, but it also gets seen; in the past four years, the films in which he has starred (Django Unchained, The Great Gatsby, The Wolf of Wall Street) have made collectively $1.2 billion at the box office. He has been nominated for an Oscar five times (four acting nominations and one for best film, as a producer on The Wolf of Wall Street). But for every certainty, there is also a rumour, a supposition, where the void of celebrity leaves a blank. He’s an environmentalist whose jet-set lifestyle leaves a huge carbon footprint. He’s a party boy, a playboy, a commitment-phobe modeliser who might, or might not, have proposed recently to his Sports Illustrated model girlfriend, Kelly Rohrbach, over dinner. With the run-up to the release of The Revenant, this sort of conjecture has reached whole new levels. Never mind the gripping subject matter – the true story of fur trapper Hugh Glass’s refusal to succumb to death after being mauled by a bear and abandoned by his hunting team – or the truly astonishing cinematography; what most of the coverage thus far has focussed on is as follows. Did Leo have fleas in his enormous beard? Did his character get raped by a bear (twice)? And is this finally the performance that will win him that elusive Oscar? The man who prowls, some 45 minutes late, into our interview gives off a very powerful message that he doesn’t want to talk about any of the above. He does this without saying a word. He doesn’t go in for small talk or niceties. He is serious, imposing even, sinking into the sofa, sighing heavily and puffing ominously on an e-cigarette as he awaits my questions. It makes perfect sense that the tiger is DiCaprio’s favourite animal (in 2013, his charitable foundation donated $3 million to the WWF to help counter the dwindling numbers in Nepal). It’s all there: the heavy-set beauty, the clear-eyed glare, the powerful, predatory silence.

Based on the performances that have made DiCaprio’s name, one could be forgiven for expecting an entirely different creature: the frivolous charm of Romeo (Romeo + Juliet), Jack Dawson (Titanic) or Jay Gatsby (The Great Gatsby); the verbose shape-shifting of Frank Abagnale Jr (Catch Me If You Can), Howard Hughes (The Aviator) or Jordan Belfort (The Wolf of Wall Street); or the complex vulnerability of Arnie Grape (What’s Eating Gilbert Grape), Danny Archer (Blood Diamond) or Frank Wheeler (Revolutionary Road). In truth, DiCaprio is all of these things and none of them. And that, of course, is his secret. “Leo is an enigma,” says his Revenant co-star Tom Hardy. “There’s something kind of magic about him.”

All those who worked with DiCaprio on The Revenant are unanimous in their praise, not only for his performance but for his commitment to a project that, during the nine months of arduous filming, pushed all of those involved to their physical, spiritual and emotional limits. “Every day was like a bear attack,’ says Iñárritu (whose last film, Birdman, triumphed at the 2015 Oscars, winning best picture, best director, best original screenplay and best cinematography).

Shot in chronological order in some extreme weather conditions (at one point, temperatures on the Canadian location unexpectedly dropped to -25C), using only natural light, The Revenant’s filming process was described by one crew member to The Hollywood Reporter as ‘a living hell’. DiCaprio, who spends the majority of the film in a state of tortured silence, did many of his own stunts; he was buried in snow, went naked in -5C weather, ate a raw bison liver, slept in an animal carcass and jumped into a frigid river. When I remind him of something he once said, ‘Pain is temporary, film is forever’, a slow smile creeps across his face. “Uh huh,” he drawls, taking another puff on his trusty e-cigarette. “I would say that this film was the epitome of that.”

From the warmth of a luxury hotel room, DiCaprio admits that the filming experience has taken on a dream-like quality, “like a big, beautiful blur”. And yet he is adamant that any suffering that he and his fellow cast and crew members went through was undertaken willingly in pursuit of a higher cause. “We all knew that we were a part of something pretty revolutionary. From the outset, Alejandro had an extraordinary vision: to create a film the likes of which audiences had never seen before. You don’t get those sort of results without going above and beyond the call of duty.”

From one day to the next, cast and crew of The Revenant could not predict what they might be up against. Quite apart from environmental factors beyond their control – frozen sets or, conversely, days and days of waiting for snow that never came – there were also logistical challenges to contend with. Because of Iñárritu and his cinematographer Emmanuel Lubezki’s absolute insistence on working with natural light, filming was restricted to an hour and a half a day, while the rest of the time was spent rehearsing and prepping shots that the director would then refuse to put on film if they weren’t absolutely perfect. “He’d be like, ‘No, no, we’re not going to shoot until the raindrops on that section of the bear fur match that section.’”DiCaprio laughs. “No, no, there’s a patch of ground there that doesn’t look quite right.” That’s what it was like. Every day. Like a master painter painting a landscape.’

“I was not the only one!” jokes the surprisingly light-hearted Iñárritu, creator of such savage masterpieces as Amores Perros, 21 Grams and Babel. “Leo, like me, is absolutely obsessed with total perfection and doing whatever it takes to get to the next level. I think we both suffer from the same disease, which is called chronic dissatisfaction.” “I get unhappy doing things that I’m not passionate about,” admits DiCaprio. “Because I feel like I’m squandering this incredible gift I’ve been given to finance films. As soon as my name alone was enough to make this happen, I vowed to myself that I was going to work with directors who were changing cinema, doing something important, you know? This goes back to when I was a teenager, feverishly watching movies like Taxi Driver and Apocalypse Now and saying to myself, someday, I’m going to be a part of films like this.”

A pretty, precocious only child – “essentially a dwarf with the biggest mouth in the world” –the young Leonardo (so-called because his German mother, Irmelin, had first felt him kick while admiring a Da Vinci painting in Florence) had always had, he says, a “strange, almost sickening desire to perform”. In fact, his earliest memory, aged three, is of going to a hippie concert with his Italian-American father, George, and leaping on to the stage to tap-dance for the restless audience who were chanting for the band to come on. Unable to focus in the schoolroom, DiCaprio flourished in front of an audience. Plus, acting seemed like a golden ticket. “Money was always on my mind when I was growing up,” he has said of his childhood, spent in a rough neighbourhood in East Hollywood. His parents, respectively a legal secretary and a distributor of underground comic books (who separated amicably when he was one), didn’t earn much money. “So I was always wondering how we were going to afford this and that,” he tells me. “Acting seemed to be a shortcut out of the mess.”

From the outset, DiCaprio’s parents did everything they could to facilitate their child’s ambitions. “I’m completely indebted to them in every single way,” he says, his guard dropping totally, and touchingly, when the subject of his mother and father arises. “They listened to me, you see. They listened to their kid saying, ‘This is what I want to do,’ and they supported me unconditionally; they made me feel that all my dreams were within reach. Now, I know a lot of people who have grown up in much better-off families, with much more solid family structures, who haven’t in any way had that level of love in their lives.”

“Leo’s humanity in all the characters he has played,” says Steven Spielberg, who directed him in Catch Me If You Can, “can be traced directly back to how close he is to his family.” One, or both, of his parents often accompany DiCaprio when he travels; two days previous to our meeting, he and his father (“the wisest, most well-read man I’ve ever met”) were photographed attending the COP21 United Nations Conference on Climate Change in Paris (DiCaprio is a UN Messenger of Peace). He and his mother (“honest, practical, real, no-nonsense”), with whom he continued to live for several years after Titanic made him the most famous young actor in the world, speak at least once a day. So obsessively does she photograph him when she is with him that he has apparently nicknamed her ‘Mamarazzi’.

When the 16-year-old DiCaprio – by now 6ft 1in after a sudden growth spurt – told his parents that he wanted to leave school to focus on his career, they supported him. And when, two years later, he beat off several other young hopefuls (including his friend Tobey Maguire) to land the role of a boy abused by his mum’s volatile boyfriend – as played by his hero, Robert De Niro – in This Boy’s Life, they made sure he knew how lucky he was. When, in 1994, he earned his first Oscar nomination (best supporting actor) for his portrayal of a child with learning difficulties in Lasse Hallström’s What’s Eating Gilbert Grape, they worked hard to stop his head from being turned. “My dad’s philosophies, particularly, have had a big influence on my career,” he says. A passionate counterculturalist, George has always steered his son towards unusual work. As a breathtakingly beautiful 19-year-old with Hollywood at his feet, DiCaprio didn’t take the obvious commercial route, choosing instead to play a drug addict in The Basketball Diaries and the tortured poet Arthur Rimbaud in Total Eclipse.

Who knows whether having this critical weight behind him helped DiCaprio to retain credibility when Baz Luhrmann’s Romeo + Juliet and, subsequently, James Cameron’s Titanic – one of the highest-grossing films of all time – catapulted him into cover-boy madness, but he certainly came out the other side. That said, ‘Leomania’, when it hit, was hard to take. On a trip to Brazil, he was recognised by rainforest Indians, in Paris a teenage girl hung on to his leg and refused to let go, and truckfuls of paparazzi followed him whenever he went out. After Titanic, DiCaprio consciously didn’t work for two years, turning down commercial projects such as Spider-Man and Star Wars in favour of more left-field, director-led films like Danny Boyle’s The Beach and Woody Allen’s Celebrity. “He wanted to prove himself, show that he was worthy of his success,” says his Titanic and Revolutionary Road co-star, Kate Winslet. “And I know that, because I felt exactly the same thing. When the bar is set that high, you have to work 10 times harder just to keep it up there.”

Never, for one moment, did DiCaprio take his eyes off his ultimate goal: working with Martin Scorsese. When rumours reached him of his cinematic hero’s development of a 19th-century gangster epic called Gangs of New York in 2001, the then 27-year-old moved agents just to get closer to the project. When DiCaprio talks about his mentor, he leans forward excitedly. “Marty is the great director of our time, who has taught me two crucial things,” he says. “One, it takes a long time and a lot of patience to make a good movie; and two, film is as valid an art form as painting or sculpture. Ultimately, like any artist, I want to make lasting pieces of art; movies that people will look at and appreciate in 50 years’ time.” In the 13 years since the release of Gangs of New York, the two men have worked together on four further films – The Aviator, The Departed, Shutter Island and The Wolf of Wall Street (the first and last of which DiCaprio was Oscar-nominated for). “He’s like a silent-film actor,” says Scorsese. “He can flash half a dozen emotions in a matter of seconds, simply by using his eyes.” During Scorsese’s tutelage, DiCaprio has turned from a beautiful boy into a handsome man. Their sixth collaboration, The Devil In the White City, due to start filming soon, will see DiCaprio taking on the role of HH Holmes, the 19th-century serial killer.

“I’ve been very lucky to have achieved a lot of the things that I dreamt of achieving as a young man,” he says. “But, at the end of the day – and I truly believe this – it is not about achieving great wealth or success. Because they don’t bring happiness ultimately. They really don’t. What matters is whether or not you’ve fulfilled the idea of having led an interesting life, whether you’ve contributed in some way to the world around you.” In the 18 years since its launch, the Leonardo DiCaprio Foundation has worked on some of the most pressing environmental issues of our day, donating more than $15 million to the protection of key species – sharks in California, tigers in Asia, elephants in Africa – and calling on world leaders to address climate change. For its founder, the global forecast is “catastrophic unless we transition to a different way of producing energy in the next five years”.

“2015 was the hottest year in global history,” he explains, “and we are actually seeing the tipping point happen right now. I witnessed it myself, first hand, on The Revenant, when unprecedentedly warm conditions in Canada meant that we had to move the whole production to Argentina in search of snow.” On his rare breaks from filming, DiCaprio – who lives in an eco-friendly apartment block in New York and drives a hydrogen-powered BMW – was not in nightclubs. He was flying around the world, working on an environmental documentary and giving rousing speeches – with the extra ambassadorial gravitas of an enormous beard – to the UN. “As an actor, I pretend for a living,” he declared. “I play fictitious characters often solving fictitious problems. I believe mankind has looked at climate change in the same way, as if it were a fiction. But I think we know better than that.”

“Leo has the grace of a really great leader,” says Hardy, who witnessed him leading by example on the set of The Revenant: never complaining, never giving anything but his all, and always thinking of others. “He aims for the stars to catch the moon,” continues Hardy, “and he’s not happy if he doesn’t achieve the highest possible standards in everything he does.” Which brings us to the question of that elusive Oscar. Surely – surely – this year will be DiCaprio’s year. What more – one can’t help but wonder, watching every skin-peeling moment in The Revenant –can one man do to get the recognition he deserves? “Seriously, if I were you, I really wouldn’t ask him that question,” advises Hardy. “One last question,” instructs the clipboard-wielder who has been hovering in the doorway for the past five minutes of our interview. DiCaprio takes a long drag on his e-cigarette and fixes me with those eyes, almost daring me to ask. When I don’t, he laughs – a big, relieved laugh – and stands up to warmly shake my hand. Leonardo DiCaprio: you might think you know him, but you almost certainly don’t.