Leto plays The Joker in the superhero film, which also stars Will Smith, Margot Robbie, Joel Kinnaman, Jai Courtney, Cara Delevingne, Viola Davis and Adam Beach.
"He sent (Margot Robbie) a nice love letter with a black box with a rat in it - a live rat. It was beautiful. Then he sent bullets to Will (Smith) with a letter," cast member Adam Beach said.
Leto, 43, also sent them a video, reported Aceshowbiz.
"He encompasses this beautiful man and he shows it in, 'Hello, how are you?,' but in an instant, he goes to psychotic behavior where you're in fear. But you love him because he has both of those genuine sides," Beach praised Leto.
The film will release on August 5, 2016.