"Let me tell you, nothing in this film is CGI," she insisted Friday on "The Tonight Show," only to be immediately challenged by host Jimmy Fallon.
"That's not true," he said. "The whole floating explosions ... cities blowing up. There's, like, a crocodile man fighting people."
OK, some of the film is CG, sure. "But, like, explosions? All real," Robbie said. "Lots of prosthetics, like Croc prosthetics." And, yes, that underwater scene.
For that, the actress trained to be able to hold her breath for five minutes.
"I worked with this amazing, like, free-diver," she explained. "He came in, and I did four sessions with him. His names Kirk -- I called him Captain Kirk -- and, anyways, he was ... like, it's all about learning to lower your metabolic rate. [...] It's all about conserving oxygen. You kind of like meditate underwater. It's what free-divers do, and it's amazing."
Robbie ended up competing with her stunt double to see who could hold their breath the longest (the stunt double won, with five and a half minutes).
"And then I watched it, and they cut the first half and the last half [of the scene], and I was like, 'But I was underwater for so much longer than that!'"
Written and directed by David Ayer, "Suicide Squad" also stars Will Smith, Jared Leto, oel Kinnaman, Viola Davis, Jai Courtney, Jay Hernandez, Adewale Akinnuoye-Agbaje, Ike Barinholtz, Scott Eastwood and Cara Delevingne. The film opens Aug. 5.