Josh Hartnett turned 30 last month. This is middle-aged in the life of a teen heart-throb, a life that will hurtle meteorically towards an early obsolescence if not rescued with judicious career planning. He is still a baby, of course, just a wise one. When you ask him about the Brat Pack, he doesn’t think you mean Rob Lowe and Emilio Estevez; for him, the term describes his friends Casey Affleck, Summer Phoenix et al, the vegetarian jeunesse dorée with ecopolitics and non-leather shoes, whom he came to London to see in the teen drama This Is Our Youth, having turned down the offer of a part himself. Along with his rock-star buddies Kings of Leon, who are playing in town tonight (he can’t go — too busy), they are the closest he gets to a gang.

He describes himself as a loner, someone who pleases himself, proud to have walked away from acting just as he was poised to make megabucks, and who turned down the role of Superman. And having starred in the blockbusters Pearl Harbor and Black Hawk Down, he wouldn’t make another war film. “I don’t think I could survive another one,” he says, referring not to the gruelling boot camps, but to his flagging self-esteem. “I have to present who I am in a much clearer fashion. I am the guy who wants to experiment with different things, who wants to be taken seriously.” Before him on the coffee table in the private members club where we meet is a battered copy of Ryszard Kapuscinski’s Shah of Shahs. “Did you read The Emperor?” he enthuses. “It’s fantastic.” Above him is a modernist portrait of George Bush Sr. “Jeez, he’s putting me off,” sighs the young man who campaigned for John Kerry.

There is something in his manner that suggests weariness, but it might just be nerves. Hartnett is in London to star in the West End production of Rain Man, based loosely on Barry Levinson’s 1988 movie, with Tom Cruise, and Dustin Hoffman as the idiot savant. He saw it once, many years ago, and has been careful to avoid it in his preparations for the role of Charlie Babbitt, the young car dealer who takes his autistic older brother on a journey across America, moving on the way from bitterness to protectiveness. “You might like him a lot less in this version,” warns the actor better known for gentle heroics. “In the film, I think he was dealt with as a kid. In this, he’s more of a threat, because he has so much hate for everybody: he doesn’t trust anyone.”

There is much mistrust attached to the business of interviewing a movie star. There are calls on your mobile from publicists in LA wanting to talk through the impending encounter (gave them the slip); there are demands that you do not ask about his private life (as if); in this case, also, that I didn’t mention the late Heath Ledger, who was a friend of his. It is mildly surprising to find, beneath this vortex of suspicion, a young man in a black woolly hat he might have borrowed from Jack Nicholson or Benny of Crossroads, sitting on his own on a sofa nursing a glass of flat cola. He rises politely to greet you, looking more like a beat poet than a glamour boy, and, indeed, his dream role is to play Bob Dylan. Hartnett is low-key, softly spoken (though not today employing the leonine purr he uses for acting and television interviews), slightly hunched into himself as he describes the problems of a mere three-week rehearsal period.

He is accustomed to the reshoots and longueurs of life on set, but not to the terror of a live audience, with lines to master, stage fright to quell, countless performances to enliven. The last time he acted on stage was probably his drama-school Threepenny Opera, where he was talent-spotted, then flown to LA. He picked up an agent and a career-launching buzz the very next day. When I ask if his fame had been intimidating to the other actors, he shakes his head, more with incredulity than modesty. “I’m the one who would have been intimidated. I’m the new kid on the block. I don’t have any history to rest on down here. I was a big question mark for everybody involved in the production.”

What’s unusual about Hartnett is his distance from an industry that normally demands total immersion for its spoils. He is the crown prince of the gossip rags and has dated the right women: Penelope Cruz, Helena Christensen and most seriously Scarlett Johansson, with whom he lived in New York; but his travelling companion is his little sister, and his real home is a mansion in Minnesota. In 2001, after he’d finished filming Black Hawk Down and just before the release of Pearl Harbor, the young star went home and very nearly never returned. “I don’t have the same agents any more,” he chuckles. “They wanted to kill me. That was their payday. But I was young and I hadn’t had a vacation since I was 17.” But the malaise was more than exhaustion. On location for BHD in Morocco, they were shooting outside a Rabat slum, surrounded by abject poverty and limbless child beggars whose families had severed their arms to enhance their poignancy. Hartnett was horrified. “I thought, ‘Jesus Christ, this is going on and we’re making a movie?’ I wanted to try to do something different with my life.”

It seems strange that an actor so enamoured of experimentation — his favourite films are Fellini’s 8½ and the crime thrillers of Jean-Pierre Melville — agreed to do two mainstream war pictures in a row, even if both depicted American military debacles. “Those two movies were completely different,” he argues. “Ben Affleck was like, ‘You’re not going to do it, are you? You just did a war movie with Jerry Bruckheimer.’ And I was like, ‘But it’s Ridley Scott. I can’t not do it.’ I’m glad I did. I’d always looked at war as being caused by the other side, but it’s people from all sides, so it became a mind-expanding experience.” As the idealistic soldier who believes the Americans are in Somalia to “make a difference”, the character suited his intensity and his family politics. The son of a musician (once a bass player for Al Green), he was brought up in the socialist propriety of Minnesota. “I wasn’t raised with the ideal of making lots of money,” he says.

During his extended AWOL, he was courted and chased; his friends hid him in the boots of their cars. “I went home to a totally different world.” He started working with charities and on the Kerry campaign. “It allowed me to figure out who I was going to be as a man.” He was eventually tempted back by the prospect of working with his childhood hero, Harrison Ford, in Hollywood Homicide (2003); and then Mozart and the Whale (2005), a true-life Asperger’s love story of a savant who — in a curious coincidence — diagnosed himself from watching Rain Man. The screenplay was by Ron Bass, who wrote the Levinson movie. Hartnett researched his role deeply, which became a balm for the shallowness of a world he had fallen into. But now the tables are turned: whereas his West End co-star Adam Godley is talking to psychiatrists, Hartnett needs to forget his informed, sympathetic approach for Charlie. “I need to unlearn all the stuff I know so that I can be a pain in the ass.”

As a child, he was a concern to his father and stepmother, rebellious, strong-minded, bright but a truant, and, quite understandably, emotionally bereft. “My mom took off when I was pretty young,” he says cautiously. “I felt as a child that nobody understood me. I went on a loner route for a long time. I didn’t listen to my father and my stepmom, which became a sort of habit, and maybe that’s why I don’t listen to anyone in this industry, either. I had supreme self-confidence. I think it came from relying on myself when I was younger.”

As a teenager, he was involved in local theatre, auditioned for acting schools because his friends were doing it, but really wanted to be a painter. “I never wanted to act, it just sort of progressed into being my career.” Hartnett is caught between gratitude for the assets that smothered a half-hearted actor in wealth and fame and a suspicion that without the looks — symmetrical face, broad manly cheekbones to match his shoulders, cushiony lips that smile only when genuinely amused, and inky-dark eyes (which, in his close-ups, do much of the work) — it might have been tougher yet more rewarding. “I don’t see myself as incredibly handsome,” he shrugs. “I don’t wake up in the morning and say, ‘Hey, beautiful’. In LA, I’m not the best-looking guy on the block. I think how I look accelerated things in the beginning, but now I want to make movies that will stand the test of time.”

His beginnings were so charmed, they are almost an insult to the struggling wannabes of Tinseltown. He likes to relate that he auditioned 14 times and failed to get a part in Terrence Malick’s Thin Red Line — which is true, but offers no clue to the otherwise seamless rise. His first role was as Jamie Lee Curtis’s son in Halloween H2O, one he was forced to accept by the Weinstein brothers, who threatened that he wouldn’t be cast in Robert Rodriguez’s The Faculty unless he complied. “I didn’t have any interest in that movie. I was made to do it.” Wasn’t he just thrilled he got the break? He shakes his head. “I never had any struggle or any commitment. I was 18. I hadn’t decided on a life of acting.”

They would have been crazy to let him get away. Hartnett has been much championed by his industry’s all-powerful moguls; as the producer of Top Gun, Bruckheimer also championed the young Cruise, a quite different proposition. Cruise wanted success so badly, he would do whatever it took, charming his way to a reputation as the politest, cleanest-cut young citizen on whom fame would ever deservedly alight. But Hartnett doesn’t want to be loved or desired or emulated; he doesn’t want to be Cruise or Pitt or Clooney. “I want something different. I want my own career. If I can parlay my success into making smaller things happen, I won’t be an actor for ever.” Maybe his recent coming of age will provide the bridge into a recognised maturity he seems to need right now. “The problem with being a teen heart-throb is that you are easy to dismiss as an actor, which irritated me.” But then he laughs at himself for the first time. “On the other hand, I’m thinking, ‘Thirty? That’s old.’ Oh my God, I’m 30. I’m freaking out.”

Rain Man previews at the Apollo, W1, from August 28

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