I got to direct Robert Downey Jr. in Tropic Thunder, and he took on a role that few would have had the courage to tackle and, I am now convinced, none could have played except him. Yes, Downey, 43, is Iron Man, but he really is Actor Man. And also, by the way, a genius. In the realm where box office is irrelevant and talent is king, the realm that actually means something, he has always ruled, and finally this summer he gets to have his cake and let us eat him up all the way to the multiplex, where his mastery is in full effect.

Now, the genius thing can be off-putting because...how do you act with a genius? How do you direct a genius? Maybe, I secretly hoped, he really isn't such a genius and I will find out that—hah!—it is all just a lot of hype, it's all charisma and editing. Wrong. First day at work—definitely a genius, playing a Method actor from Australia playing an African-American soldier in an over-the-top Vietnam War movie within a movie and never breaking character (in the movie).

He riffs, he improvises, he is funnier than the script most of the time. What it takes to be an actor, good or bad, is a bravery of sorts. Downey's choices are as brave as they come. He is not afraid to try something fully, knowing it could end in disaster but also understanding that that is where greatness lies. Many actors do this, but so few with the underlying sensitivity and facility with their emotions that he has.

I was an admirer of his before I worked with him. Now I feel as if by some gift from a higher power, Robert Downey Jr. was sent down to Earth to help us all realize (through his work) that the human experience is a sad, funny, beautiful thing, full of imperfection and irony. I have had a chance to work with one of the greats, and he tricked me into actually thinking I was keeping up.

Stiller co-wrote, directed and stars in Tropic Thunder, which comes out in August