Leonardo DiCaprio was just 17 when he got his first big part, in This Boy's Life. "I had the big meeting with Robert De Niro," DiCaprio said. "I read a scene with him. To make an impression, I yelled some of the lines in his face. I was a smart ass in a lot of ways. Very over-confident."

One day, Robert De Niro and Ellen Barkin were doing a scene, while DiCaprio cavorted about, distracting them. Ellen started lecturing him: you've got to learn to behave to be more like the two of us.

"Like the two of you," DiCaprio shot right back. "Let's see, on the one hand, he did Raging Bull. On the other hand, you did Switch. And you're the one who's telling me what to do?"

DiCaprio and I are sipping lemonade at the Chateau Marmont, a few blocks from where he grew up in Los Angeles, discussing his latest film, The Beach. For DiCaprio, it is a change of direction. The film was, he says, "the first film where I really wanted to take more responsibility for what I was doing as far as steering the story in the right direction. And Danny [Boyle, the director] was unbelievably open to my doing that."

What was the hardest thing about The Beach? Well, DiCaprio was badly stung by jellyfish during the water scenes. He tried wearing tights to protect himself but they didn't help much. And he worked 70 days with only four days off.

And what about Titanic? What was the hardest thing about that?

"Everything. Every day. The hardest thing I've ever had to do was make that movie."

Physically? Emotionally?

"Everything."

"He gets in quite a lot of fistfights," says Cheryl Rixon Davis, co-owner of the Playroom nightclub in LA. "But he's very friendly. Very sweet. People only mention it because it's him. He's a perky chap, a feisty young man. The fights are broken up quickly. No one wants to see Leo beaten up in their club. For him it's part of the entertainment."

What about girls?

"A different girl every half hour. He comes in with Carmen Electra. They make out big-time to entertain the crowd. He's an exhibitionist like she is."

DiCaprio's fame has left him wanting something real. Something pre-Titanic. "It was so unreal to me when it all happened," he says. "It was almost like a joke." A part of him longs for a world untouched by movies. Especially his movies. In his new movie, Leo's character Richard doesn't want to see the beach overrun by tourists. In his new life as a megastar, Leo himself doesn't want to be overrun by fans.

"People slag him off for Titanic," Danny Boyle tells me. "But you have to try to separate him from the publicity, that crazy balloon."

"What's the nature of his appeal?"

"He has a feminine side that all great actors have access to. It's what makes women fancy him."

"There was a moment when I was at an airport somewhere," says DiCaprio. "This girl grabbed my leg. It was like one of those things I had seen in a Beatles documentary. She was this fanatical fan. I wanted to say to her, 'Look, it's just me. I'm really, truly a regular guy. You don't need to do this.' But there was nothing I could say.

"My attitude to fame was some thing different. I was going to be who I was, and I was going to lead my normal life and do what I was going to do no matter what people say."

How has that worked out?

"The more you try to do something like that - well, the more you have to compromise."

Does he think he's grown up a bit since Titanic?

"Certainly. There is obviously a whole adjustment process. I have learned to trust myself more. I think I take more responsibility for what I need to do. I really try to control a little more where I want things to go. But I would not want to become an adult in every sense of the word. Who the hell does?"

DiCaprio has just bought a house in Hollywood, which suggests growing up. I tell him about a scene a friend of mine once witnessed: Greta Garbo and Montgomery Clift having a loneliness contest. Garbo said: "I am so lonely that I only ever have one dish in my dishwasher." Clift said: "I'm so lonely that I only ever have paper plates."

DiCaprio interjects immediately, "I'm so lonely that I don't have any dishes at all."

Lonely? Does that mean he doesn't have a girlfriend?

That's right, no girlfriend - "Nope" - at the moment.

Later we watch The Beach together in a preview theatre. Afterwards I tell DiCaprio I liked it very much.

"Really?" he asks.

He seems to lack confidence. He knows this movie, this career move, is risky. His character begins light and grows steadily darker. Will his Titanic fans like this very different DiCaprio? Surely not all of them. But the movie works. It is about something. It says something. It is part of a straight line that goes from This Boy's Life to What's Eating Gilbert Grape to the Basketball Diaries to Total Eclipse to Romeo and Juliet to The Beach. Titanic is the aberration.

• The Beach opens on February 11.