Leonardo DiCaprio isn’t an actor who just says yes to any new movie project, but Clint Eastwood managed to secure his talents through the promise of mystery.

Clint Eastwood is a magnet for success, but it was the “mystery” of the role he offered Leonardo DiCaprio in his 2011 drama movie that drew the Oscar-winning actor to the role, rather than the allure of Eastwood himself. The two’s first and only collaboration was for 2011’s J. Edgar, a biographical movie about the life and career of J. Edgar Hoover and his pivotal role in establishing the FBI.

The film was markedly divisive upon its release, with some critics saying that Clint Eastwood and DiCaprio had made one of the best movies of the year, and others criticizing its narrative choices. To an extent, that was always to be expected when attempting to make the best drama movie about the secretive and complex figure, whose personal life was subject to so much speculation.

And, that’s exactly why DiCaprio decided to take on the leading role of Hoover himself, in what is one of the best Clint Eastwood movies from the past few decades of his directorial career. “When I can’t immediately define the character, and there’s an element of mystery to it and still a lot to be explored, that’s when I say yes,” said the actor, speaking with The New York Times about the role.

“I like those kinds of complicated characters. I just do.” DiCaprio then explained how meeting Cartha D. DeLoach, one of the only people still alive at the time who knew the real J. Edgar Hoover, allowed him to dig more deeply into the character he inhabited for the screen. “I wanted him to tell me how he walked, how he talked, what his hands looked like, what his desk looked like, what was above his desk,” said the Titanic star.

“The research of these roles is half the fun and half the challenge — maybe more. It’s what makes it exciting to me,” he concluded. That kind of commitment and dedication is what makes DiCaprio one of the best actors of his generation, and it shines through in his portrayal in the movie.

Concurring with the sentiment, Eastwood explained his own thoughts on DiCaprio’s performance. “Leonardo could make a lot of money making mechanical genre pictures, but he wants to be challenged. And it’s much more of a challenge to play someone who doesn’t have the slightest thing in common with you.

Now go see how Clint Eastwood taught DiCaprio how to fight.