The TIME 100

In the early '90s, my friend Bob De Niro made a picture called This Boy's Life. He told me, "I worked with this kid. He's really good. You've got to work with him someday." The "kid" was Leonardo DiCaprio.

Then I saw What's Eating Gilbert Grape. One of the hardest things for actors to do is play someone who's mentally challenged. Very often, no matter how good they are, you can see the machinery at work. With Leo, in this performance, I didn't see the machinery. He wasn't playing the role. He had allowed the character to become a person and then to inhabit him.

I was impressed. Then when I saw him in The Basketball Diaries as Jim Carroll, in Total Eclipse as Rimbaud, as Romeo for Baz Luhrmann, as the young hero of Titanic, I realized that range wasn't an issue.

He was the actor among a generation of gifted young actors. Once he became a superstar, he had a choice. It would have been so easy for him: he could have made a comfortable home for himself in commercially viable, action-thriller roles. But he chose to go down a different, more adventurous path.

We worked together for the first time on Gangs of New York, and that was when I first witnessed his commitment to his chosen craft—a commitment that is fierce and total. Unlike many other actors, Leo will keep pushing until he breaks through to an emotional and psychological truth, no matter how ugly or childish or inexplicable.

He holds the screen, like a great silent actor. With his face, his eyes. He knows how to use them and how to use his body; he has an innate sense of how much to give and how much to hold back. I look at him, and I know he's a true actor. The camera knows it. And most important, the audience knows it.

Leo, 32, is, at heart, fiercely private. He's also responsible, in the sense that he takes his citizenship—of the world—very seriously, as seriously as he does his acting. He has used his celebrity very carefully in an effort to raise awareness, particularly among younger generations, of the environmental perils facing our planet. And like all serious artists, Leo is a passionate student. He never stops studying and learning.

The "kid" has grown up to be a man I'm proud to know and whom I feel lucky to count as a collaborator. We've made three pictures now. I can't wait until we work together again.

Oscar winner Scorsese has directed more than 25 films