Ethan Hawke cannot get Boyhood out of his system. He will not let it go. Richard Linklater’s sublime meditation on growing up, starring Hawke and Patricia Arquette as separated parents and Ellar Coltrane as their son, was shot every summer from 2002 to 2013. This year it was released to universal acclaim and became even more a part of his life. “For years I tried to talk to people about it and nobody had any context of it,” he says. “You try to explain to somebody: ‘Hey, I just did this great week of shooting in Austin.’ ‘Oh yeah, what is it?’ ‘Story about a kid.’ ‘When’s it gonna come out?’ ‘Nine years.’ Oh. So I think what will be lonely and sad is the feeling next year, when it really is over. I really felt like we stumbled on to something really new. And I know I can’t return to this moment.”

Over the course of 12 years, Boyhood became an intrinsic, inseparable part of Hawke, and he of it. Today, we’re ostensibly discussing Predestination, a new time-travel transgender thriller, which I tenuously link to both Boyhood and Linklater’s 18-year-spanning Before trilogy (charting Hawke’s character Jesse’s ongoing relationship with Julie Delpy’s Celine). All of these films, I suggest, are about how much we change while fundamentally remaining the same. “Yeah,” he says. “I don’t know if it’s because everything is refracted through Boyhood right now for me, but... what I love about Boyhood is that spanning these 12 years, you see that the characters at the end of the movie are so obviously different than they were at the beginning, and yet they’re still clearly the same people. It gets at that essential question of what part of ourself is continuing.”

Hawke has changed a lot over the decades, and he has remained the same. At 18, he was the wide-eyed new kid on the block in 1989’s Dead Poets Society; at 24, via 1994’s Reality Bites, he became the poster boy for Generation X. He’s played heavyweight, gritty roles in 2001’s Training Day and 2007’s Before The Devil Knows You’re Dead, and more recently has been showing up in obscenely profitable multiplex fare such as 2012’s Sinister and 2013’s The Purge. Yet his youthful sincerity and the pensiveness he exudes have remained throughout.

Now 44, he shows no sign of an ego during our hour-long conversation and is constantly reflective. His latest project is Born To Be Blue, a drama about jazz trumpeter Chet Baker. It’s set in 1969, when Baker was 40, ravaged by drugs, and had lost the ability to play after having most of his teeth punched out. The film centres on the one restorative element of Baker’s life at the time, his relationship with girlfriend Diane Vavra. “It’s extremely bittersweet because it’s the last period he was ever sober,” says Hawke. “He temporarily could have salvaged some part of his life. But no sooner did he learn to play again than he took on all the same habits.”

Personal experiences have drawn Hawke to the story. “He was just a really beautiful, soulful person who wasn’t in control of his habit,” he says of Baker. “And for me, personally... I’ve lost two of the great heroes of my life to heroin. Philip Seymour Hoffman [with whom he acted in Before The Devil Knows You’re Dead] and River Phoenix [co-star of Hawke’s debut film Explorers] are two of the greatest actors of my generation. It’s very rare that you really would think of an actor as an artist, but River, even though he lived such a short period of time... he was a poet, you know. He was the real thing. And so was Phil. Both of these guys were real inspirations to me, and it’s really sad... I’m just trying to understand all that.”

Hawke with co-star Philip Seymour Hoffman in Before The Devil Knows You’re Dead. Photograph: Allstar

Hawke, it appears, has never found himself in big trouble. He attributes his level-headedness to his early experience on Dead Poets Society, with director Peter Weir and co-stars who prized acting over partying. After the film, Hawke formed a theatre company with some of them. Also, of course, there was Robin Williams who, after Dead Poets, got Hawke his first agent, and would come along to his theatre productions to support and encourage.

“One of the things that was hard for Robin was...” Hawke says, before pausing to think. “He wasn’t ‘normal’. He was a comic genius, and I don’t use the word lightly, he really was. I was 18, but you couldn’t spend a tremendous amount of time with him and not know that life was difficult for him. Every outburst... every up has a down, right? You see it. It’s so hard for people like Robin who have these explosive highs and all this joy, but no balance in their life. And I’d hoped that he had beaten it. But he was such a beautiful person to all of us young guys.”

Hawke met Richard Linklater through his theatre company. Linklater came to a play, collared Hawke and told him about Before Sunrise. Although Hawke was fresh from the success of Reality Bites, he admits he was still learning to act. By 2005 and sequel Before Sunset, however, he’d gone toe-to-toe with Denzel Washington in cop drama Training Day, for which he received an Oscar nomination.

Of late, Hawke, a lifelong genre fan, has wanted to mix things up further, hence the increasing amount of sci-fi and horror on his CV. In 2009 he starred in Daybreakers, a lurid vampire thriller by Australian twins The Spierig Brothers. Their follow-up, Predestination, adapted from sci-fi author Robert A Heinlein’s 1959 short story All You Zombies, is an even bigger bowl of madness. It concerns fate, free will, time-travel, a forced sex-change operation and a serial-killer manhunt. It’s impressively insane and a lot of fun.

“It’s absolutely out of its mind,” says Hawke, who plays a “temporal agent” flitting through time. “I love science-fiction that’s not oriented around action or effects. Whether it’s Robert Heinlein, Kurt Vonnegut, Philip K Dick, HG Wells or whoever... that kind of mind-bendy science-fiction where you can really attack themes in a new way. And when I read Predestination it was like: ‘What the fuck did I just read?!’” He particularly likes the fact that although the lead character is transgender, the film is not explicitly about transgender issues, it’s about all of us. “There’s something about Predestination that actually does get at identity, for me,” he says.

Talk of time and identity moves back to Linklater, and to the overlaps between Hawke himself and his characters. “The kind of ethos Rick and I were going for in the Before trilogy was absolute pulverisation of the line between character and actor,” he says. As a child of separated parents, and then a divorced father himself, how much did he draw on his own familial experiences for Boyhood and Before Midnight (in which Jesse has kids by different partners)?

“I don’t know...” He pauses. “It’s a little less intellectual than that. There’s that great Mark Twain line about the point of art being to alleviate shame. The more you can be telling the truth and the less you facilitate all the stupid lies...” For a couple of minutes, he talks about how Linklater’s films make us feel, as opposed to more fantastical movies that make us feel our lives are boring. “The thing about Rick’s movies is they make you feel like your life is really exciting, compared to these motherfuckers,” he laughs. “My point is that I have always, especially in the safe hands of Linklater, tried to mine the material that’s been interesting to me. I was a child of a divorce and I’m a parent of divorce. And it’s been a giant roaring dragon of my psyche, so it feels good to write about it. And it makes me feel good to see a crowded theatre of people watching a divorced family and to see that be part of a narrative where there’s nothing wrong. The goal of Before Midnight was: can you make a romantic movie about a couple of 40-year-olds who have been together a decade, and have it be romantic, and not tell one lie? Can you do it? To do those kind of exercises you have to mine your own life. It’s just the only way you’re gonna stumble on anything real.”

Hawke with co-star Julie Delpy in Before Midnight. Photograph: Allstar

Whether or not they make a fourth Before film is in the hands of time. They may get the urge again in another few years (they film them every nine), and they may not; Linklater has said Before Midnight feels like a satisfying conclusion. It would be a shame if they didn’t return for 2022, though, when Hawke will be 51. What’s it like to have these characters always subconsciously percolating at the back of his mind? “I’ll give you an example. The other day I had to go to the doctor because I can’t see any more. So I was getting fitted for glasses and I thought: ‘OK, we’ll do another Jesse and Celine; Jesse’s got glasses! It’ll be a great little touch to put in. Any time I have a life-changing thing, however subtle it might be, I see it through the context of how it might affect Jesse and Celine.”

Whatever the future holds, Hawke is enjoying a huge career peak right now. Has he taken stock of this critical high? “You never really know where you are,” he says, countering the question by discussing perceptions of success, and how those we think are succeeding are often failing, and vice versa. “So I’ve never had that feeling of security. No sooner does something go well than I think: ‘Well, are the best days behind me? Is it over now?’”

He laughs: “I just try not to think that way.”

Predestination is in cinemas from 13 Feb