"It was the most unbelievable scene I have ever been party to," James Cameron says of the night more than 2,000 hysterical bodies charged the director.

No, Cameron wasn't fending off a mutiny among extras and crew on the set of his $200-million "Titanic." The setting was the movie's world premiere last month in Tokyo, where Cameron and wife Linda Hamilton experienced a near-lethal dose of Leonardo DiCaprio fever.

The details seem a fitting end to the turbulent production of "Titanic"--and an omen of what may lie ahead for its 23-year-old star DiCaprio, whose charismatic work in "William Shakespeare's Romeo & Juliet" earned him adulation at home and especially overseas.

'Linda and I were in one car," Cameron recalls. "Leo was in another with some friends. As we approached, I kept getting calls: 'We've brought in an additional 150 police in riot gear, the hundred we had weren't enough--don't come any closer.' Finally they said, 'You absolutely cannot come in the front. There are 3,000 fans here.'

"So we decided Linda and I would come in the front. When they saw a limo, they'd congregate there and Leo would slip in the back. And it worked. The only problem was, Linda and I almost got killed.

"Three thousand people surged toward us. Somebody recognized Linda, a scream went up, everybody assumed it was Leo, and they converged on us. It was like being in a riot in Calcutta. And police couldn't control it."

Amid screams of "Romeo! Romeo!" the 6-foot-tall heartthrob took the stage before the screening and made an impromptu speech; the filming of "Titanic," DiCaprio told his ecstatic fans, "made a man out of me."

Reminded of his words a month later in Los Angeles, DiCaprio slumps behind a corner table in a dimly lit Italian restaurant, shakes his straw-blond hair and smiles a wan "here we go again" grin.

"And boy, have I heard that friggin' quote over and over and over again!" he says with a sigh, his voice still high enough to suggest adolescence.

"It's one of those spur-of-the-moment things that suddenly pops into your head. You say it to hear the Japanese girls go, 'Wooooo!,' when you use the word 'man' in a sentence. And all of a sudden, it defines you as a human being."

Fair or not, DiCaprio is being defined plenty these days. "Titanic" (which opens Friday) will bring him his widest audience, and his performance--as an American-artist Romeo to Kate Winslet's Juliet aboard the fateful liner--is likely to solidify his place atop Hollywood's most wanted list.

It's also likely to boost him, willing or no, to a more surreal level of fame and personal scrutiny.

"It's not something I'm entirely ecstatic about," DiCaprio says evenly, swirling imaginary circles on his placemat. The lids over his brownish green eyes hover at half mast after an evening photo shoot, and he's still recharging from making three films back to back to back.

"I'm just going to have to deal with the consequences, which isn't the worst thing in the world. It's not people attacking you with butcher knives--it's people wanting to speak to you, get a picture or an autograph.

"Maybe now I have to be a little more conscious what I do. But I won't ever be a weird recluse." A moment later, he asks the writer not to mention the restaurant's name, since it's in his neighborhood.

He also politely declines to discuss his dating life, but does own up to at least one live-in companion--a pet dragon lizard.

For four years, Leonardo DiCaprio has been struggling to balance celebrity, acting, and yes, encroaching maturity, since his electric turn in 1993's "This Boy's Life," as a troubled teen abused by stepfather Robert De Niro.

"I got that part because I was a little ignorant," DiCaprio says. His rosy mouth curls into an irresistible smile. "I wasn't afraid of De Niro during the audition process. I sort of stood right up in my chair, pointed in his face, and did things that I thought would get their attention. And it did."

(De Niro, he adds, is "the guy I will always measure everything to--the first masculine cinema figure that I've experienced firsthand. We'd do a lot of violent stuff [in the film] and he was completely supportive and caring.")

Within a year, before his 20th birthday, DiCaprio earned Oscar and Golden Globe nominations for his work as Johnny Depp's retarded brother in "What's Eating Gilbert Grape," bringing to the role an exuberance that bypassed the usual movie disability cliches.

But just when mainstream stardom seemed likely, DiCaprio opted for quirkier roles, like the hedonistic gay poet Rimbaud in "Total Eclipse" and heroin addict Jim Carroll in "The Basketball Diaries." The latter film, released in 1995, recently brought DiCaprio his only brush with controversy; 14-year-old alleged Kentucky gunman Michael Carneal reportedly claimed a scene in "Diaries" inspired him to shoot eight students.

Even DiCaprio's mainstream movie choices had an edge. In the Sharon Stone western "The Quick and the Dead," he was a cocky gunslinger killed by his father; in "Marvin's Room," he was Meryl Streep's pyromaniac son.

By the time Fox's 1996 MTV take on "Romeo & Juliet" brought DiCaprio box-office clout, he'd been pegged as a screen natural (he's never formally studied acting) whose prankster nature could switch on a dime to the heaviest requirements of film acting.

"Leo has a great ability to turn it on when he's on the set--he was just born with it," observes "Titanic" co-producer Jon Landau. "He has an inner quality that conveys honesty. And he makes people around him feel so comfortable."

Adds Cameron: "What he does is sort of a bait-and-switch. He doesn't seem to be serious at times. But when he's kidding on the set, really he's processing on a much deeper level."

DiCaprio did no shortage of processing before signing on to "Titanic," three months after reading Cameron's 165-page treatment and while filming Baz Luhrmann's "Romeo & Juliet" in Mexico.

"Traditionally Hollywood has spun out such garbage when it comes to larger-budget films," he explains. "On 'Titanic,' the formula [for success] was there. It's a serious subject, a director with a great track record . . . but at the same time, a film like this changes your life.

"And it's a big risk. If it doesn't work out, it's a lot of time and energy. I just basically didn't want it to be a big . . . a big. . . . " Struggling for erudition, he surrenders to instinct. "You know, piece of [expletive]!"

Encouraged by Luhrmann, DiCaprio agreed to do a reading for "Titanic" in Los Angeles with Kate Winslet.

"Kate wasn't cast at the time," Landau remembers. "After they tested, she said to us, 'No matter what you do, if you do not cast me, you have to cast Leo.' "

But DiCaprio still had doubts, partly about the character of Jack, the impetuous artist who wins a steerage ticket on the Titanic in a card game, and whose life force drives the romance.

"I just wasn't used to playing an openhearted, free-spirited guy," he admits. "I've played the more tortured roles in the past. It was difficult to be someone closer to 'me' than anyone else.

"And to tell the truth, I haven't really wanted to do love stories at all. I've never sort of . . . liked them."

But Cameron had seen the spark he was after. DiCaprio was his choice, over such rumored finalists as Matthew McConaughey and Chris O'Donnell.

"I jokingly say I auditioned Leo for 15 minutes and he auditioned me for three months," Cameron recalls. "Leo is the kind of guy who goes to the hardest thing. He wants to prove himself against a difficult test. I think his initial view was that Jack wasn't hard enough to play.

"I had to point out to him that when you don't have a [character with] a drug addiction, a physical or mental affliction, these overt actions--almost props--you have to rely on something more inward and subtle.

"To be charming and charismatic every moment on the screen is a much bigger challenge. It's the thing Jimmy Stewart did when 10,000 [others] couldn't. When Leo realized the difficulty was a 10.0, it became more appealing to him."

Of course, that Olympian score also applied to the shoot. In the fall of 1996, DiCaprio reported for duty on the deck of the Titanic, built 90% to scale in Rosarito, Mexico.

"We filmed four months before Christmas, then we came back thinking we'd last two or three more weeks. We ended up staying 3 1/2 months. I've never had to have that much physical patience with anything.

"Some people were on that set for 10 months. To see the deterioration of them . . . their spirits were just drained."

DiCaprio's got a fortuitous boost from his co-star, British actress Winslet. He insists their relationship was never romantic, but rather a close friendship, forged by surreal adversity.

"We were joined at the hip. There was a moment when Kate and I were on the poop deck, which was on hydraulics. We looked down and there were, like, 20 men clinging onto the rail below us on bungee cords.

"And when the poop deck went to its peak, the guys jumped off and started bouncing off each other, bouncing off girders. [Three stunt men were injured on the shoot.] Then you looked up and saw, like, 18 cranes with huge lights shining on you, and Jim Cameron coming from a little spot in the sky, zooming in past your close-up to the people diving below you.

"Kate and I looked at each other. Our eyes just bugged out, and we said, 'How did we get here? How did we get to this moment in time?' "

To fight off tension and boredom, DiCaprio turned to video games ("they've saved my life on a lot of movies"), practical jokes (on his last day, he dumped a bucket of ice water on Cameron) and his gift for mimicry. "He can do a killer impersonation of De Niro," Cameron recalls, "and I'm sure he does one of me that has everyone in fits."

So didn't such rambunctiousness irk Cameron, whose obsession with detail and alleged temper are legendary? Apparently not.

"Leo kept himself and the set amused," the director explains, "and I think it aided the on-screen chemistry between him and Kate. On the set, she tends to be more serious. Leonardo would pull some grotesque prank and get her laughing in spite of herself.

"Kate was totally enamored of him, celebrative of him as a person, and that crept into the scenes. I don't think Leo was doing that consciously to help the film. That's just how he is, how he dealt with the massiveness of the enterprise."

Asked in turn about Cameron, DiCaprio pauses, choosing words carefully.

"Jim is an interesting, complex guy. What I will say is, he has a side . . . you know . . . sides he has to represent being this type of director. When you have a thousand people asking you different, complex questions, you have to be a general.

"The other side of him is completely sensitive. Not a lot of people realize that even in something like 'True Lies' or 'T2,' there are interesting character things going on. Jim let us improvise entire scenes, even though he wrote them. At other times he was dead-set on what he wanted to do.

"He's not the easiest director to work with, most definitely. But I'm not going to sit here and say that he's not somebody I'd work with again. I probably would. And this movie went beyond my expectations."

Few babies are so bold as to help choose their own name, but Leonardo DiCaprio is the exception.

The story goes that back when he was in the womb awaiting his first premiere, his parents were strolling through a gallery in Italy. As they studied a DaVinci, the baby kicked--so they christened their future son Leonardo.

"They're the people I trust most in my life," DiCaprio says of his parents, who divorced before he was born and whose hippie lifestyle helped shape his nonconformism.

"They're the coolest parents in the world. I feel proud to have grown up with people like this to look up to."

George DiCaprio is an ex-New York performance artist and underground comics distributor who introduced Leo to friends like Robert Crumb, Charles Bukowski and graphic artist Robert Williams. DiCaprio's German mother, Irmalin, was born in a bomb shelter during World War II; she left her homeland at age 11, and met George during college.

The couple moved to Hollywood, thinking it was the exciting center of Los Angeles. Instead, recalls their son, "they wound up by Le Sex Shoppe and the Waterbed Hotel.

"It was definitely a rough neighborhood, but it was cool. I got to see a lot of stuff. It's good to grow up like that, I think. It's good to see that side at a young age."

Despite his attraction to tortured souls onscreen, DiCaprio insists his childhood was a happy one. A born impressionist, "it didn't occur to me 'til I was 13 that you could actually make a living being an actor. I thought it was a unique club you had to be in."

That changed when his stepbrother, Adam Ferrar, appeared in a Golden Grahams cereal commercial and earned $50,000. DiCaprio's competitive side kicked in, and soon he was acting in educational films and TV spots. "My first commercial [in 1988 at age 14] was Matchbox cars. I played a young Mafia lord with a new Matchbox suitcase collectors set."

A role during the 1991-92 season on TV's "Growing Pains" followed, then "This Boy's Life," which came out in 1993. DiCaprio credits that film's director, Michael Caton-Jones, with teaching him "the basics of how to survive in this business . . . what you need to do on screen, how you focus. He was my first class in how to make a film.

"He also wanted to keep me a little naive to the whole process, so he could get natural reactions out of me."

DiCaprio's been achieving those natural reactions ever since--but it's easier for him to do than analyze.

"I don't really understand the process. The main thing is just getting into the reality of what the character is, finding all the suitable things to go along with it. I often look at a situation from the outside, like I was a camera.

"People asked me if I did research on what the Titanic was. But all Jack really knew was, he was getting on a big ship to America. What I dug about him was the artist side--he was a bohemian figure, into the ashcan school of art, which was all about painting people for the reality of who they were."

The resonance between fictional artist and the actor playing him is hard to miss.

Recalls James Cameron: "When I was talking about Jack to Leo, I said, 'Jack is somebody who can shake your hand and Xerox your soul.' I think Leo can do that, too.

"He may not have studied acting in a formal sense, but he's had ample opportunity to work with some of the best actors. He's an absolute chameleon and a study of human behavior."

Since completing "Titanic," DiCaprio has moved on to three roles in two films. In early 1998 he'll be "The Man in the Iron Mask," playing both Louis XIV and his imprisoned twin; coming to the rescue are Four Musketeers: Jeremy Irons, Gerard Depardieu, John Malkovich and Gabriel Byrne.

He's also completed a cameo in Woody Allen's next untitled project.

"I did four days on it," he says. "It's a really funny character--a spoiled young Hollywood actor that's completely self-absorbed. Arrogant and a punk. Kenneth Branagh's character is trying to sell me a script and I'm pulling his leg."

It's a part that sounds like what DiCaprio could have become, with his mercurial talent, good looks and the lures of fame. And there's no question he enjoys celebrity.

"What's cool is basically going to different countries and getting everything paid for," he volunteers when asked what he enjoyed in Tokyo. "Getting your friends to go with you. . . . It's not bad at all."

He's also acquired a reputation as an inexhaustible club-hopper. But DiCaprio insists that image is overblown.

"Leo is an actor first," echoes Cameron. "Some young actors lose their grounding in their work, but I don't think Leo will ever lose that.

"He's going to make interesting choices. He's going to be seduced by the dark side, the more difficult characters. He'll be all over the sky. Every time they think they've got him pegged, they'll miss him."

DiCaprio surprised some recently by turning down Ang Lee's next drama, "To Live On."

"I'm exhausted," the actor explains, adding that Lee is "one of my top three favorite directors." The role was taken by one of DiCaprio's closest friends, Tobey McGuire ("The Ice Storm"): "I'm happy," DiCaprio says. "If my boy can do it, all power to him."

So what will DiCaprio be doing instead?