Being a successful, Oscar- nominated actor isn't enough for Ethan Hawke. When he's not appearing in films or spending time with his wife Uma Thurman, he writes fiction; his second novel, an offbeat love story, focuses on intimacy and marriage.

Ok: maybe watching Gattaca on video wasn't the best way to prepare for an interview with actor Ethan Hawke. As any movie buff will tell you, Gattaca is a stylish sci-fi thriller in which an awesomely well-groomed Hawke shimmies around concrete-and-glass walkways accompanied by the stunning Uma Thurman, his wife in - as they say - real life, glamour oozing from every pore. It's a terrific movie, but it does make you wonder if your wardrobe - not to mention your IQ, your ability with interior design and your skin care routine - can quite cut it. In real life.

Happily, at 8.50 a.m. on a Saturday morning in Dublin city, Ethan Hawke's resemblance to the vision of loveliness which appeared in Gattaca is approaching absolute zero. Instead, he is crouched over a low table, his hair all over the place, packing away a full Irish breakfast with the zeal of the true carnivore. Rashers, sausages, black pudding, mushrooms, tomatoes - you name it, he's demolishing it. When introductions are set in train he jumps up, wipes his hands down the front of his jeans, offers one to shake and declares: "Hey, I like your pants." There's not a trace of a pout or a hangover or a celebrity ego.

"You know," he muses, wiping his plate with a sausage the size of a small dachshund, "doing this press stuff isn't good for your head, really. All these questions about you. All these people who have their own take on who you are, and what you are. You begin to think the whole world revolves around you."

The sausage vanishes. "Makes you question your own identity, too. But, man, is that the best breakfast I've ever eaten, or what?"

Probably best known for his performance as Todd in the über-weepie, Dead Poets' Society, Hawke has an impressive list of films to his credit, is co-founder of a theatre company in New York, and is the author of two reasonably well-received novels, the second of which - Ash Wednesday - he is here to promote. It is an offbeat love story featuring a feckless young protagonist, Jimmy Heartsock, and his pregnant girlfriend, Christy - Hawke himself describes it as "a meditation on intimacy and marriage".

So why did he call it Ash Wednesday? Is this marriage as Lenten punishment? He grins. "There was something about the whole experience of getting married that reminded me of Ash Wednesday - you know, the whole thing of 'till death do us part' and 'dust to dust' and this idea of 'OK, now you're supposed to be serious'."

And the wedding scene in the book, with an edgy Jimmy cracking inappropriate jokes as he and Christy queue for their marriage licence at City Hall - is it by any chance based on Hawke's own wedding to Thurman in 1998? "No, no, no. Not really," he says. "My wife was raised a Buddhist; her father is Robert Thurman, who is one of the pre-eminent Buddhist scholars in the US and was the first white monk in the Dalai Lama's order. So our whole wedding was heavily influenced by, I mean, we had to deal with a lot of that."

He corrects himself. "I had to deal with it."

As a general rule, he says, religion is something he enjoys thinking about - one of his favourite writers is the Catholic mystic, Thomas Merton - "but I'm not qualified to speak about it, you know?". With its road-trip structure, constant shifts of perspective between the two protagonists and short, staccato sentences, Ash Wednesday is a cinematic novel - but it's a novel none the less, and a well-crafted one at that.

Why did Hawke want to write a novel in the first place? "I don't know," he admits. "When the idea for the story first occurred to me, I thought maybe I could make a movie out of it - but I've never been able to write a screenplay. A screenplay is kind of a blueprint, or a guidebook, you know? Like a guide on how to sail the Atlantic or something - 'you gotta go here, make a left there' - but when I sit down to write, it just comes out this way.

"You're so much more free in fiction. One minute a guy's simply driving, listening to the radio, and suddenly you can digress to when he was seven, and what he's really thinking about is how this drive relates to that. And the other thing is, I've been involved with movies since I was 13, and it's really liberating to be involved in something else. To meet this whole publishing world, to make your brain work in a different way - I mean, I've been doing the same job since I was a kid."

Sounds as if writing might be an escape for him. "It's a complete escape," he says. "Although if you say it's an escape, there's an implication you're not taking it seriously - and I worked really hard on this book. I think it's important if you're gonna publish something, and ask somebody to pay some money for it, that you put some energy into it so it's worth their time and money.

"When I first got married, it would irritate my wife that I would write, because she didn't know what I was doing. She thought I was going to the movies - and sometimes I was. But as we've been together longer, she really wants me to do it because if I do a little bit of that, even a couple of hours a day, I'm just easier to be around."

With a daughter of four, a son of seven months and a wife who is also an actor, it can't be easy to create the sort of time and space necessary to write a novel. Hawke shrugs. "I don't know what anyone else's home life is really like," he says. "Really. I mean, you know how it seems, but . . . is it chaotic? Sometimes. Sometimes it's boring. It's difficult, certainly, if both people are aspiring to have some participation in the outside world. You have to work hard to maintain some kind of balance; but if you can find some kind of balance, it's an ideal situation.

"It's also a great way to parent, to show your kids that you're good at something - and not even that you're good or not good, but that you take it seriously and enjoy it, that you have a life of the mind, you know? That's a valuable thing to teach your children."

Hawke, it seems, takes everything pretty seriously - except his movies. Ask him about them, and he laughs and lets fall tiny drops of gossip. Alive, the story of the Andes plane crash? "Making a movie about people eating each other was very strange," he says. "I lost 30 lbs for that movie. Don't ever go on, like, a crash diet with 85 other young men. It's not amusing."

Snow Falling on Cedars? "I know, I know - it's like a silent movie, isn't it? But when we filmed it, it had lots of dialogue. They just cut it all out. Which, as an actor, was kind of frustrating when you were watching it, because you were like, OK, this scene has now got music over it, and that scene is just, you know . . . and, sure, it's poetic. It's exquisitely beautiful to look at. But all I did was sit there and look anguished."

And is there a new movie in the pipeline? He makes an impatient gesture. How to get good roles, that's the problem.

What? Ethan Hawke, bright young thing, Oscar nominee for his last film, Training Day? Surely the scripts keep rolling in?

"Just think about it," he says. "Of all the movies that come out every year, how many do you really love? Would you say maybe 10 is high? OK, so there's 10 movies that you love - that's of those that come out that you're interested to see. If you're an actor, maybe - and I'll go high again - maybe there's four of those 10 that might have a part that you'd be right for. And there's about, I dunno, say, 35 really good young actors at any given moment. So you'd be lucky to see one of those parts, you know what I mean?

"I've been acting all my life; it's not just a thrill for me to be in a movie any more. I've been in, like, 20-something movies. I've gotta love it. So instead of reading a bunch of banal scripts, you start wanting to write a book. Can you see the thought process?"

I can see his name on at least on more book jacket, that's for sure.

Ash Wednesday by Ethan Hawke is published by Bloomsbury at £14.99 sterling