He couldn't stop laughing on set.
Karen Fukuhara, who plays Katana in Suicide Squad, claims that Leto never responded to her questions or greetings on set. "Everything [was] like, 'Ha. Ha. Ha. Ha. Hi, pretty.' I'd be like, 'Hi, I'm Karen,' and he'd be like, 'Haaaa.'"
He sent Smith a box of bullets and a letter
"Jared went 'full Joker,'" Smith told E!. "And the rule is never go 'full Joker.'"
He sent Margot Robbie a love letter and a rat
“At first I was like, 'This is disgusting,'" Robbie said on The Tonight Show. "It jumped out at me, so I got a fright and got off to a rough start." Despite co-stars urging her to dispose of the animal, Robbie gave it a home tricked out with a rat-sized playpen, slide, and hammock. She also bought a leash in order to walk the rat on set, which Jared Leto probably loved. The rat now lives with director Guillermo del Toro, apparently.