Joaquin Phoenix blows into London at the same time as the snow, like a competing weather front, talking up a blue streak. Outside, the flakes are flying and the temperature is below zero. Inside, he is preaching peace, love, tolerance and understanding – and it is all I can do to get a word in edgeways. My questions sit unread on my lap; the publicist hovers anxiously at the door. Who’s going to stop him? Who has the clout? When the man’s on a roll, it’s difficult to say: “Cut!”

“Just be in the moment,” Phoenix advises at one point. “Don’t overthink it, let it be what it is. If you keep trying to find what’s unique in the moment, then the danger is that you miss that very thing.” I think he’s talking about the craft of film acting. He may be talking about life.

Phoenix has been a turbulent screen presence for so many years that it is startling to realise that he is only 43. He has played dented Johnny Cash, the depraved emperor Commodus, an introverted lonely heart in Spike Jonze’s Her and a raging, rough beast in Paul Thomas Anderson’s The Master. Some great actors are deliberate and precise, but Phoenix is at his best when he seems on the outer edge of control; when he threatens to break loose from the picture and bring the scenery crashing about his ears. The man goes at things freestyle, for better or worse. He says: “The great thing about film is that you get to make mistakes.”

Phoenix in his new film, You Were Never Really Here. Photograph: Allstar/FILM4

On his latest film, You Were Never Really Here, he has found a kindred spirit in the British film-maker Lynne Ramsay – another wild talent who sometimes courts disaster. Ramsay bashed out the script on spec, at speed, after bailing out of another movie (Jane Got a Gun) on the first day of production. She tells me she wrote the lead role with Phoenix expressly in mind. “Stuck his picture above the computer, as though I could telepathically put him in my film.” Sure enough, the actor materialised on set having never met her before. “He’s instinctual and unpredictable,” Ramsay says. “The range of stuff that he gave me … I could have made several other completely different films.”

For Jesus, what makes his death such a sacrifice is that he didn’t want to die

As it is, her picture is roiling and delirious; a missing-person thriller spun violently on its head. Phoenix plays Joe, a traumatised former soldier on a mission to retrieve a trafficked teenager. He says he did some research – spoke to an ex-military guy who does similar work. Mainly, though, he followed his gut. “Lynne sent me an audio file of Fourth of July fireworks. She said: ‘That’s what’s happening inside Joe’s head.’ That’s one thing that really clicked for me.”

So forget about Joe; how about playing Jesus? In Mary Magdalene – a revisionist take on the gospels released later this month – Phoenix co-stars as the messiah alongside his girlfriend, Rooney Mara, who plays Mary Magdalene. Surely this was a role that required some rigorous research? There’s a lot of material to wade through about Jesus.

He shrugs, unconcerned. “Lots of material. Lots of conflicting material. But, in the end, it’s a character. And, as with all characters, whether it’s Johnny Cash or whoever, you have to make it about a man; about his personal experience. And for Jesus, what makes his death such a sacrifice is that he didn’t want to die. This was a man who wanted to continue the experience of living, just as we all do. So it was important to me to find those human qualities.”

Phoenix as Commodus, with Russell Crowe in Gladiator. Photograph: Moviestore Collection/Rex

Phoenix preaches the gospel of living in the moment and it could well be that he is experiencing one right now. Later this year, we’ll see him as a wild west assassin in Jacques Audiard’s adaptation of The Sisters Brothers, and a quadriplegic cartoonist in Gus Van Sant’s Don’t Worry, He Won’t Get Far on Foot. His career has never looked better; he should be relishing the glory. It’s just that, the way he tells it, films only matter when he’s on the set – after which they may as well not exist. What excites him is the process of acting – of losing himself in a role, of losing track of time – the same as it did back when he was a kid.

He recalls his first time in front of a camera as though it happened yesterday: “Instantaneous joy. The most enjoyable thing. For some kids, it’s the first time they crack a ball or score a goal. For me, it was this. I was eight years old, and I remember the first scene on the TV set so vividly. And I knew that I loved it – the physical sensation; how powerful it was. That’s the feeling I’ve been chasing ever since.”

At the time, he was one of five children: a band of pint-sized performers; Haight-Ashbury von Trapps. The Phoenixes had alighted in California with their footloose hippy parents and started busking on the streets. He says: “My mom got a job at NBC. And then through her boss we were introduced to an agent. She was the only agent who would take all five of us because my parents didn’t want us to be split up.” He snorts. “We were a very expressive, creative bunch.”

The leading light of the group was his big brother, River; clear-eyed and handsome, a superstar in the making. Joaquin (then named Leaf) was smaller and darker, seemingly destined to play second fiddle. Times were good; the money rolled in. The family may have been seen as a band of eccentrics, but, it transpired, this was no bar to success.

“Well, we’re vegans,” he explains. “And when you first start acting, what you mostly do are commercials. And we said to our agent: ‘We’re not going to do anything for McDonald’s or Coca-Cola. We’ll do bicycles and dolls, and that’s about it.’ And our agent was like: ‘This is lunacy. First of all, it’s already impossible to break into this business and now you’re excluding 70% of what you might be able to do.’” He laughs at the memory. “I guess we were either bold or stupid.”

I have the sense that there have been times when he has grown disillusioned with acting, but he insists otherwise. Nonetheless, his career has been marked by sudden absences, abrupt reinventions; periods when he appears to have lost the plot. After River’s death from a drugs overdose in 1993, Joaquin dropped out of sight for a year. In 2009, he caused a furore when he lumbered on to the David Letterman show, apparently blasted and inarticulate, to confirm his retirement. “I’ve been working on my music,” he mumbled into his beard. He added that when he returned, he would come back as a rapper.

In Paul Thomas Anderson’s The Master. Photograph: Sportsphoto Ltd/Allstar

This turned out to be another Phoenix performance, a stage-managed stunt he sustained for a year. It resulted in a rambunctious faux-documentary, I’m Still Here, directed by his then brother-in-law Casey Affleck. The film portrayed a spoilt brat in freefall – snorting cocaine off a groupie’s breasts and being defecated on by his personal assistant. Even when it premiered at the Venice film festival, the suspicion persisted that it might all be true.

“The film started as a running joke,” he says. “It was like an SNL sketch: how does a 35-year-old retire from acting? But it was a separate character; it wasn’t me. Then I thought it would be funnier if I played myself. In sitcoms, the character is always named after the actor, right? It’s Jerry Seinfeld in Seinfeld, Ellen DeGeneres in Ellen. Or it’s Larry David in Curb Your Enthusiasm. So I thought: ‘Let’s take that idea and make a really fucked-up, perverse version of it – like an R-rated sitcom.’ But then it got out of control. It just went everywhere.”

He concedes that the lines became blurred. “Certainly, that was a period when things felt very stressful for me. I wanted out. I wanted my fucking life back. So, yeah, there was a bit of crossover there.”

Our culture tells us over and over that punitive justice is good … it’s in every goddamn movie

If the aim was to shake him from his rut, the gamble paid off. Phoenix emerged from I’m Still Here like a man reborn to play twisted, volatile Freddie Quell in The Master (still, for my money, his best screen performance). But the film has a darker legacy, too. Producer Amanda White and cinematographer Magdalena Górka subsequently accused Casey Affleck of various instances of sexual harassment – one of which allegedly occurred in an apartment he was sharing with Phoenix. Both lawsuits were settled out of court. But I wonder if he feels that mistakes were made and lines crossed. To put it another way: did the anarchic spirit of his and Affleck’s production foster a culture in which sexual abuse could occur?

“I don’t know how much I can talk about that without getting into details that I’m prevented from talking about,” he says. “I can’t talk about the details of those lawsuits, which is unfortunate because I think you know that I’m somebody who likes to talk freely.” He weighs his options; comes up dry. “I think I can’t talk about it without running the risk of causing more trouble.”

OK, then. Let’s talk more generally. Does he feel that the US film industry has an in-built sexual abuse problem? “Obviously! How could I not? It’s rampant and it’s been going on a long time.”

His infamous appearance on David Letterman. Photograph: CBS via Getty Images

Could he, personally, have done more about it? “I’m sure,” he says. “At its core what we’re talking about is an abuse of power. And sex is a weapon. And I have seen abuses of power all over. There are many things that I either wish I had been aware of at the time or wish that I’d had the strength and conviction to stand up in that moment and say: ‘Hey, stop!’ I feel that I do that at times. But, yeah, there are other times when I probably don’t.”

Even so, he believes that the ground may be shifting. “I wonder if we’ll look back on this time the way that my parents experienced the anti-war movement or civil rights. When it started, I was super-cynical. I thought: ‘Well, we’ve just elected a president who talks about grabbing women by the pussy.’ So I didn’t have a lot of faith. But I think this may be a moment of radical change.”

I wonder whether Trump’s presidency may actually be a catalyst. The man’s politics are so brazen, so unashamed that they at least put the debate centre stage. We know what the stakes are. We can see what we’re fighting.

Phoenix winces. “Possibly,” he says. “But I don’t like you saying ‘fighting’. When people want to fight violence with violence, it defeats the purpose. My mom runs a peace-building organisation [the River Phoenix Centre for Peacebuilding]. What they do is restorative justice. Both parties – the aggressor and the victim – have the opportunity to talk about things. That can only be valuable, right?”

As Johnny Cash in Walk the Line. Photograph: Everett/Rex Features

He gulps a breath. “Our culture tells us over and over again that punitive justice is good. And it’s in every goddamn movie, right? Every movie is the hero saying: ‘You killed my wife and now I have to go and kill you.’ Which is also its own fucked-up thing. The beautiful wife being killed or kidnapped. But so much of it comes down to the language we use. If you say: ‘Trump, you’re a greedy, narcissistic asshole. Fuck you! I’m going to battle you,’ then you become the very same energy you have problems with. It’s OK to feel anger. I guarantee you that Martin Luther King and Jesus felt anger. But it’s important to have empathy. I don’t believe people are born evil.”

We have overrun wildly. The publicist is practically kicking down the door. Still, I’m interested in what he says about his mother’s organisation. How does he feel his own career measures up?

“Ha,” he says. “If you’re asking what’s more important – my work or my mom’s – then clearly it’s my mom’s work. But I do think the arts are really important to a culture. It gives me a platform to speak out, for a start. It has also probably helped my mom have this organisation. It’s named after my brother. So it’s all connected, right?”

The photographer is waiting for him down the hall. The actor, confounding as ever, promptly bounds off in the opposite direction. “I have to pee so bad!” he shouts. And with that he’s gone; the door slams shut at his back. Even the messiah needs the odd bathroom break.

You Were Never Really Here is out on 9 March. Mary Magdalene is out on 16 March.