6. He suffers from obsessive-compulsive disorder.
Like the man he portrayed in The Aviator, the eccentric Howard Hughes, Leo had OCD as a child. “I had the thing where I would walk to school and have to walk, you know, go back a block because I didn’t step on a crack,” he told Katie Couric in 2004. “And I felt like something was going to go wrong if I didn’t do that.” But the star has been able to talk himself out of his obsessive urges: “I’m able to say at some point, ‘OK, you’re being ridiculous, stop stepping on every gum stain you see. You don’t need to do that.”