The first time I met Ethan Hawke he was lying on the floor of his hotel room.

It was during his press tour for Before Sunset, the second and best of the movie trilogy about a star-crossed romance that he and co-star Julie Delpy conjured with director Richard Linklater. It was 2004, shortly after Hawke had split from his wife Uma Thurman amid accusations that he'd had an affair. Answering probing questions from just about every journalist who entered the room had left him spent, knocked out, his eyes red with emotion.

It didn't help that his character also contemplates cheating on his wife with Delpy's French student, the girl he'd met nine years earlier on a train bound for Vienna. "The way to make a movie like this interesting as a work of art is to blur the lines between performer and character," he told me. "My favourite kind of writing and my favourite kind of performing is when you can tell it cost somebody something. That's why this movie will always be very dear to my heart. It was never a job."

Loading article content

It's what makes Hawke such an intriguing actor. Described by one interviewer as "the anti-Tom Cruise", his performances plumb his soul. They cost him something. Maybe not in that Method actor way, subsuming his personality to become the character, but rather he draws from his own deep well of experiences - blurring those lines. Usually, it's come with Linklater, the director he's worked with eight times, who describes their relationship as like that between "comrades, like blood brothers".

Their latest collaboration, Boyhood, is arguably their masterpiece, even greater than the trilogy of Before Sunrise, Before Sunset and Before Midnight which has twice seen both men, and Delpy, nominated for an Oscar for best screenplay. Shot in 39 days over the course of 12 years, Boyhood tells of a Texas family, seen through the eyes of a lad named Mason (newcomer Ellar Coltrane). In his immediate sphere are his sister Samantha (Lorelei Linklater, daughter of the director) and mother Olivia (Patricia Arquette), who begins the film by separating from the children's father, Mason Snr (Hawke).

What follows is quietly remarkable, as the film subtly lets us watch Mason - quite literally - grow before our very eyes. There are no title cards, telling us what year we're in, just clues - Game Boys and iPhones come and go and songs on the soundtrack (from Coldplay's Yellow to Daft Punk's Get Lucky) tune us into the passing of time, as Mason moves from six to 18, from innocence to experience. Rather than a film about big events, it's a beguiling evocation of life, capturing the infinitesimal details that make up personal growth - conversations, experiences, emotions.

Revisiting actors annually for a dozen years is unprecedented. The only project that comes close is Michael Apted's Up documentary series, which revisited a group of Britons at seven-year intervals. But never has a fiction film attempted to incrementally watch its characters grow. "I've been making movies since I was 13 years old," says Hawke, "and I have never …" He stops, lost for words. "We didn't even know how to make deals for the movie. You can't be contractually obligated to do something for more than seven years. It was such a … it was an experiment, really."

Rather lamely, I call the film unique. Hawke lets out a wry laugh. "What's strange is the overuse of vocabulary. So many people say: 'Oh, this is a unique project' - this one actually is a unique project. To see the story of a family told using 12 years … it's never been done before. Never in my life have I done anything that isn't like something else." He cites Training Day, the corrupt cop drama that won him an Oscar nomination in 2002. "Well it's kind of like The French Connection, you know what I mean? This is kind of like nothing else."

While the film is primarily about Mason's evolution, it's just as fascinating to watch the adult characters change over time. Hawke's character is always there in his son's life, even as he starts a new family. "He makes some big lifetime decisions. It's really the story of a guy who decided to show up as a dad, and had to put [to one side] his dreams of being a musician, or being a more maverick-like figure … I think you watch him grow up. I think you watch him become a man, this father, in some ways good, and in some ways not. It's like life - that's what I like about it."

Hawke claims he never had any doubts about Linklater's idea. "To be honest with you, when he told me, I felt sure it would work." All he had to do was make commit to shooting a few days each year; the most golden of handcuffs. "Every time I was doing a job, before I took it, I'd have to call Rick and say, 'OK, hey I'm thinking about playing Macbeth on Broadway, so rehearsals would start here, and is there any way I can shoot my section before previews start?' I've been doing that for 12 years."

Remarkably, Hawke was 31 when Linklater suggested the idea. He was still married to Thurman. Their first child, daughter Maya, was three years old. And their second, son Levon Green, had just been born. A dozen years later, Hawke's emotional life has somewhat mirrored that of his character. He's been through a divorce, remarried and become a father twice more, after new wife Ryan Shawhughes gave birth to two daughters, Clementine, now five, and Indiana, two.

It brings it back to this idea of art costing something from its performer: Hawke has never been afraid to plunder his own existence like an emotional magpie. "In a lot of ways, this whole movie is a collage - of Patricia, and mine and Rick's experiences as parents, and funnelling our memories and feelings as teenagers into Mason and Samantha, and using what their experiences are right now and turning it into this piece of fiction. As a parent, we all revisit our own childhood, when you watch your kids go through this."

But what about Hawke? Can he pinpoint how he's changed over time? "When you're young you're so naturally curious … and I would like to keep that quality. But if you get your hand burned a few times, you get less curious about the fire. It's a natural instinct. We all like to hope that we're growing wiser, stronger and better. I believe that in some ways I am. Over any lengthy period of time like that, I always think of life like a sailboat. It's not a straight line - you can veer to the left, come back to the centre and veer to the right - but hopefully you're moving forward."

Physically, the film lets us see Hawke age gradually (having met him six or so times over the decade, it's something I feel I've seen close up). Fortunately, time has been kind to him. He may have lost the almost boyish sheen to his features that we glimpse at the outset of Boyhood. But, bar a few grey hairs, he's weathered the storm well. Today, he's still striking: bright blue eyes, a brush-like shock of brown hair and a face that has resisted the ravages of middle age. His cheekbones are defined and sculpted, his skin taut; he can still rock a goatee too.

Yet, now 43, Hawke admits he's been hit by your typical male midlife crisis. It happened just after he turned 40. "It seemed kind of arbitrary but I think I went into a mild state of panic," he reveals. "I've always looked at myself as such a student. My self-image was one of a student and a learner, and I felt like I was supposed to be done being a student, and I should now know something. And I went into a panic that I didn't know anything. And I didn't know how to stop being a student. Then I realised I don't have to." At least he didn't buy a Ferrari.

This notion of being a perpetual learner, you might say, came from his parents. His mother Leslie, later a charity worker, and father James, who went on to work in insurance, were students at the University of Texas when he was born. They separated before he turned four, and Hawke went to live with his mother. Leaving Texas, they relocated several times before settling in New York. When she remarried when he was 10, Hawke and she shuffled out to New Jersey.

In high school, he aspired to be a writer - an ambition that eventually came to fruition, with Hawke publishing two novels, The Hottest State in 1996 (which he turned into a film, 10 years later) and Ash Wednesday in 2002. "In a way, I am a construction of my mother's imagination," he says, noting that it was she who wanted him to be a novelist. Clearly a free soul, his mother moved to Romania more recently, working for Gypsy rights and starting up an organisation - inspired by a child beggar - called the Alex Fund.

As a youngster, Hawke was intrigued not by literature but by acting. He made his stage debut when he was 13 in a school production of George Bernard Shaw's Saint Joan - kickstarting a highly regarded theatrical career that peaked, according to his mother, with a 2007 Tony nomination for Tom Stoppard's trilogy The Coast Of Utopia (ever the pessimist, she said: "You'll never do anything better"). But at least, when he was a teenager, she let him explore this urge to act. He was 14 when he secured his first film, Explorers, directed by Joe Dante and co-starring a young River Phoenix.

The film flopped, due in part to its release being overshadowed by Live Aid, and Hawke didn't act for another four years. When he did, it was as the prep school pupil inspired by Robin Williams' poetry teacher in hit movie Dead Poets Society. It's not hard to imagine Hawke's shy student as some kind of alter ego. A voracious reader and a distant relation to playwright Tennessee Williams on his father's side, Hawke twice enrolled in New York University's English programme, but his early success meant he kept being enticed with film roles. "It seemed silly to pursue anything else."

After playing a slacker in touchstone generation X drama Reality Bites, Hawke's hipster credentials were established, then cemented a year later in his first Linklater-Delpy collaboration, Before Sunrise. Never mind the anti-Tom Cruise; he was like Brad Pitt in miniature - a leading man with a character actor's curiosity. His choices were bold, experimental, such as the 1997 science-fiction tale Gattaca, where he first met Thurman. "A movie like Gattaca would never get made right now," he laments.

As a leading light in US indie cinema, Hawke has seen a disturbing sea-change over the past 15 or so years. "We thought it was tough in the 1990s because it wasn't as good as the 1970s, but now I realise that the 1990s was a wonderful time period. It's just got harder and harder and harder. For everyone. I feel sorry for young people." All of a sudden, films like Boyhood are an endangered species. "What qualifies now as a well-done art house movie is a well-done Batman movie," he says with a sigh.

Hawke cites a Milan Kundera essay that describes how corporations have "usurped cinema as an art form, in a way that it never was able to do with literature or music". You sense in Hawke that he wants to be considered a true artist, but, like most, he struggles. "Someone once said, 'One or two people a generation get to be completely true to themselves and be commercially successful.' Bob Dylan is as sincere an artist as Emily Dickinson; but one has a million dollar record contract and plays Madison Square Garden, and one was never published in a lifetime. They're at odds with each other, art and commerce, in a certain way."

While he's never been an actor to flirt with the blockbuster crowd, he's made his mark lately in genre films "to try to keep working", as he bluntly puts it. "It's the only way to participate in the commercial part of the business." Following a turn in the remake of John Carpenter's Assault On Precinct 13, he's appeared in vampire plague movie Daybreakers, serial killer chiller Sinister and social anarchy thriller The Purge. And it doesn't stop there: he's recently signed on for hip horror director Ti West's In A Valley Of Violence and has completed the lead in time-travel thriller Predestination.

Some actors might view these modern-day B-movies as below them, but Hawke is smart enough to realise that this is what's making money. Sinister took £45m ($77m). The Purge took even more: £52m ($89m). Both cost around £1.75m ($3m) to make. "A good genre movie has a great punk rock feeling to it," he says. "I always liked early John Carpenter films. My first movie was with Joe Dante, who'd done The Howling. My mentorship was in Roger Corman genre movies. What you can do with a genre movie is similar to what you can do with a comic."

Thankfully, these lucrative lower-profile films have meant Hawke's personal life isn't quite as microscopically picked over as it was when he was with Thurman. Back then, while he was making serial killer flick Taking Lives with Angelina Jolie in Montreal, he was photographed alongside Canadian model Jennifer Perzow, sparking rumours of infidelity. The fallout was ugly, but he says he's not bitter. "If I wanted to resent the media, and be unhappy about that, I'd quit acting. Or just do plays. If I just did plays for the next 10 years, I would never be in a tabloid again. So you have to realise that in some way I'm participating."

Even when he and Shawhughes got together, the couple were considered fair game; after all, she'd been a nanny to Hawke and Thurman's children - though there was never any suggestion they'd forged a relationship while he was still married. "Whether you're me or you, gossip hurts people's feelings. But it's just gossip," he says. "It doesn't matter if it's on a national level, or it's your best friend talking to your co-worker, it's irritating when people are passing judgment. Even if you've done something wrong."

Yet as a father of four, he couldn't be happier. "It's changed me so much," he says. Like Mason Snr in Boyhood, who takes his son to meet one of his old friends who is still pursing a music career, you suspect Hawke will encourage his children to follow their dreams. "There are these different paths that life can take you," he says. That might be true. But Ethan Hawke was never going to veer from his. n

Boyhood (PG) opens on July 11.