Around halfway through I'm Still Here, the 2010 documentary chroniclingJoaquin Phoenix's short-lived rap career and apparent retirement from acting, he undertakes a shambolic press junket, snapping when a journalist asks if it's all a hoax. "It's hard not to get offended, because you're talking about my life," barks Phoenix. "As if my life's a fuckin' joke to you."

It's moderately disconcerting, having recently watched that sequence, to be here in a hotel suite with Phoenix, another journalist talking about his life. When I enter the room, though, he's standing. His hair long from filming Paul Thomas Anderson's Inherent Vice, he's seemingly ego-free, loose and engaged, joking around from the off. Throughout the interview there's never a sense that he's humouring me, trotting out answers or overtly plugging anything, even the film he's here to plug

In Spike Jonze's Her, set in a near future LA, Phoenix is Theodore, a despondent, solitary writer whose life picks up when he falls in love with Samantha, a portable, artificially intelligent operating system who provides more than he could have hoped for. Can Phoenix sympathise with the character? Does he ever talk to Siri? "I swear about three weeks ago was the first time that I ever tried to talk to Siri and it didn't work out well; I think I might have said a couple of nasty things and maybe she was offended," he says. "I remember seeing sci-fi movies as a kid and they had video telephones, and I thought, 'I'm definitely doing that when that happens.' Now I have Facetime and maybe I've used it twice."

So was it Her's exploration of relationships that appealed to him more than the technology issues it raises? "It was both, and also just the idea of how subjective love is. I thought there were these really interesting ideas. When I saw the cut, I immediately had the instinct to talk to somebody else about it. It's that kind of movie, it inspires these thoughts in you. It's rare to have a movie where you wanna connect with somebody else and communicate some of the ideas."

Nobody does inner turmoil better than Phoenix, who's excelled at angst ever since his troubled teen in 1989's Parenthood, and he's exceptional in Her. Samantha is audible but invisible, so Phoenix's physical responses speak for both of them, and Jonze has said he cast Phoenix because he needed someone as compelling as him to make that work. The deal was sealed after Jonze saw a softer, more vulnerable side to the actor in the unlikely setting of an interview on the David Letterman show, and Phoenix is very much the lost puppy in Her. Some of the film's strongest scenes involve Theodore's failed marriage – he doesn't want to let go, even if he knows he should – and when the final nail is hammered in, his face crumbles only subtly, his eyes sinking into heartbreak.

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"Yeah, you know… I've never had one of those relationships," says Phoenix when I ask if the scene struck a chord. "I've never had one which ended that I felt heartbroken and consumed by. The closest I've got is getting my ego bruised a little bit. But I've never had the experience of, 'That was somebody I really loved and I'm not done.'" He's really never been heartbroken? "No. I've had unrequited love. I've definitely been like, 'I love you,' and they don't feel the same. But I've never been in a relationship and been heartbroken. When I arrived [to start filming] I think it was obvious that I hadn't really given myself over to heartbreak. I remember those first couple of days very distinctly as being a battering, like, 'Fuck!' Just really trying to get into the space of what that was, and it being something that was a bit foreign to me, because part of me was, 'Well, fuck it, come on, pick yourself up, go do something, you'll be fine!' Maybe that sounds really weird." Maybe. He's lucky. "Yeah. It'll happen," he says, not entirely convincingly.

Jonze had seen I'm Still Here before casting Phoenix. That film began as a joke, a sort of character sketch, he says, but its director, his friend Casey Affleck, wanted all or nothing. Phoenix, having committed to the project, submitted to the director's wishes. And there's much substance to what he did; he never seriously planned to abandon acting, but hewas bored. The conceit was a lie founded on truth, and that four-year hole in his IMDb list, beginning not long after he won a Golden Globe forWalk The Line, is real. Was it something he felt necessary to do in order to rejuvenate himself?

"Sounds like a good thing to say to an interviewer: 'I did this to rejuvenate myself!'" he says, springing to life, turning up the volume. "In some ways that was the idea, or the hope. I don't know if I had a fuckin' plan, it was just: 'I don't wanna do it the way I've been doing it, I wanna try something fucking different.' It's funny bringing up Spike because the great story from Being John Malkovich is Malkovich doing a scene, then Spike saying, 'Um, I don't think John would do that.' And we had that experience all the time. I would almost argue that the character [in I'm Still Here] is as much Casey's invention – and maybe even more in some ways – as mine."

He talks of being jaded by the repetitiveness of film sets, of learning lines and hitting marks. "I just wanted to do something where I didn't have the safety net, the opportunity to do multiple takes all the time," he says. "We knew I was going to retire and I was gonna try to do a certain thing and I was going to get smacked down. That's all we had. And that was really an exciting way of working: not knowing what somebody was going to say."

Even after the veil had been lifted, the message still remained, I suggest, that he isn't a gameplayer, that he marches to the beat of his own drum. "I don't know about that. You like to think you're being true to yourself but I'm sure I've fuckin' schmoozed!" he laughs. "You go to premieres and you shake hands and go round. I wear a fuckin' suit that I wouldn't normally fuckin' wear. You'd like to feel that you're not that, but you are in some ways. But I feel like you're going after something that I'm not answering. I'm just trying to be truthful because I also know that sometimes those stories can get spun the other way, 'He's on the edge! This guy, he says no to the system!' And they love pumping up that person, but he's not totally that way, just like that other guy's not totally the fuckin' kiss-ass. Do you know what I mean?"

do. In fact I feel foolish for ladening him with grand attributes. I'm Still Here was, in part, a reaction to what he and Affleck see as the mythologisation of actors, which is precisely what I'm doing with this talk of rejuvenation and non-gameplaying. Still, Phoenix is the man who, in 2012, called the awards circuit "total, utter bullshit," a dog and pony show he wanted no part of. And I'm Still Here has by his own admission informed what he's done since. He's specifically working with directors who give him leeway to explore spontaneity, to get lost in the moment. Some of his more unnerving scenes in Paul Thomas Anderson's The Master were improvised, including the bit in which he smashes up a toilet, while Jonze gave him similar freedom. As far as I can tell this wasn't always the case.

"Yeah, there are certain movies I've done because I felt it was the right thing to do for my career," he says. "That's the reality. You go, 'Fuck me: I don't necessarily want to do this movie, but I don't dislike it, and there are some interesting parts…' And I convince myself a little bit. Unfortunately that's the truth."

My assumption is he wouldn't do that now. "Who knows?" he says. "I'd like to think that also, I really would. But I don't know. I'm changing; I'll be a different person in five years. In five years the kind of movies I want to make might be totally different. I just want different experiences. Everyone tries to cast you in your public persona, and… I like action movies. I like superhero movies. And there might be a time when I wanna do something like that. It's just, I haven't wanted to. And then it becomes, 'Oh, he's this kind of actor.' I'm not that kind of actor, I'm just gonna follow what my interests are, and it might be this kind of film and it might be something different, I don't know. Does that make sense?"

I'm Still Here reminded Phoenix how it felt to have genuine frisson on camera and that's all he really wants: to get lost in it all. "The kind of actor I want to be is a child actor," he says as we wrap up. "Children don't have all the shit that we have, years of interviews, self-evaluation, thinking about yourself." For someone who would prefer not to evaluate himself, though, Phoenix is a generous interviewee. Last month he even dutifully attended the Golden Globes for his Her nomination. That said, he wore a cardigan instead of "a fuckin' suit".

Her is out on in the UK on 14 Feb

http://www.theguardian.com/film/2014/feb/08/joaquin-phoenix-her-spike-jonze