The latest Hollywood drama is the pair's third film together -- but the first time they've shared a scene!
Robbie and Pitt were both in 2015's The Big Short, and were of course co-stars in Quentin Tarantino's Once Upon a Time in Hollywood in 2019, which won Pitt a Best Supporting Actor Oscar. However, Babylon is the first time they've had scenes together, which Robbie recently told ET's Kevin Frazier was a "brilliant" experience.
"It's technically the third film we've appeared in together, and we've never acted together until this film," she shared. "So that was a wonderful gift.... He's so, so wonderful. Watching Brad Pitt, the biggest movie star, play the biggest movie star it's just something like, inherently fun."
In Babylon, set in 1920s Hollywood, Pitt plays on-type as Jack Conrad, one of the biggest movie stars in the world, while Robbie stars as Nellie LaRoy, an unpredictable up-and-coming starlet searching for fame.
"I love playing characters who have very extreme reactions to things, who are very reactionary," she shared. "Nellie is extremely reactionary, like, if you come at her with a bit of heat, she'll come back with fire."
"That's a fun and exhausting character to play," she continued. "The trick was being able to keep up the energy levels all the way through, because like, after a couple months of being on that level and knowing that you still have huge sequences to go, that's when you really gotta have a hard word with yourself and be like, "Don't slack, don't even give an inch less than what you gave day one of this shoot."
Robbie's character, she explained, is just one player in the "insane" time period of Hollywood that Babylon showcases as an extravagant world of drugs, excess and unbridled creative energy.
In fact, Robbie shared, Babylon's ostentatious opening party scene took two weeks to shoot -- and the cast felt every minute.
"It was like going to a party that crazy, but for two weeks," she shared. "It was like, the longest bender any of us have ever been on sober."
"You don’t even have to look that far to find these stories," she added of the Hollywood excess found in the film. "You get like a glimpse of just how insane and reckless, and then also you see how many people died young at that time."
As for the scene where Pitt's character dares her own to fight a snake, which was filmed with both real rattlesnakes -- "they were pissed to be there" -- and animatronics?
"It was absurd and insane," Robbie recalled with a laugh. "I mean we literally were out in the desert, it was a sh*t show in like, the best way. It was so Babylon."
Babylon is in theaters Dec. 23.