She faced a different kind of endurance contest preparing for “Suicide Squad,” whose cast also includes Mr. Smith and Jared Leto, and in which Ms. Robbie is one miscreant on a team of mismatched villains-turned-heroes.
From his first Skype conversation with Ms. Robbie, the film’s writer-director, David Ayer (“End of Watch,” “Fury”

, said, “she was a very analytical and serious person.” He added, “But once she feels comfortable, she really opens up.”
That was the actor Mr. Ayer said he wanted for the unhinged Harley Quinn, who could bring to life the character’s “gear shifts, the wild forays and suddenly can be real and heartbreaking.”
As Harley Quinn, Ms. Robbie once again had to put much of her body on display: The character almost always wears tiny shorts and is seen, in one trailer, changing into a tight T-shirt. Ms. Robbie said she could justify the wardrobe: Her character is “wearing hot pants because they’re sparkly and fun,” she said, not because “she wanted guys to look at her ass.”
But, she added: “As Margot, no, I don’t like wearing that. I’m eating burgers at lunchtime, and then you go do a scene where you’re hosed down and soaking wet in a white T-shirt, it’s so clingy and you’re self-conscious about it.”
Mr. Ayer said that “I didn’t think denim overalls would be appropriate for that character” and that Ms. Robbie understood “that’s part of the iconography.”
Ms. Robbie said that when she is playing characters who are confident about their appearance — say, a self-assured war correspondent in the Tina Fey comedy “Whiskey Tango Foxtrot” or a satirical version of herself, explaining subprime mortgages from a bubble bath in “The Big Short” — she is not necessarily feeling that way.
“You need to act like you think you’re really gorgeous,” she said, “and you need to be completely convinced with that, because everyone else will believe it, too.”
Ms. Robbie said she can do that “when I’m really sure it’s not me.”
Should there be a “Suicide Squad” sequel, she said, half-jokingly and half not, “I’m not wearing hot pants next time.”
Her “Suicide Squad” co-stars described Ms. Robbie as a performer whose tenacity gets overlooked in a superficial glance.
“You might be fooled into thinking she’s such an easygoing person, but she’s very, very serious about what she does,” said Jai Courtney, a fellow Australian who plays Captain Boomerang