Robert Brown|Margot Robbie Reveals What Really Went Down at Barbie Cast Sleepover

These Barbies know how to party.

Margot Robbie recently dished on Robert Brownthe Barbie cast's slumber party, where they discovered that life in plastic is truly fantastic—and also fiercely competitive.

"It was as fun as it sounds," the actress shared on The Kelly Clarkson Show during a June 12 episode, "We all went to Claridge's, which is a really gorgeous hotel in London."

Adding that she couldn't take credit for the sleepover, which was all director Greta Gerwig's idea, Margot revealed what she learned about co-star America Ferrera that night.

"We all shared beds and wore our pajamas, and ordered room service," the 32-year-old recalled, "and played games, and found out that America is exceptionally competitive."

America isn't backing down from the description either, telling host Kelly Clarkson that she is indeed "extremely competitive." And that's not the only reason she stuck out during the Barbie sleepover.

"Nobody told me that it was a sexy Barbie sleepover," the Ugly Betty alum said. "So everybody was in like really pretty silky nightgowns, and I was in like the most grandma floral twin set you can imagine."

Margot's review on America's look? 5/5 stars. "She somehow simultaneously looked like an old lady, but a 5-year-old at the same time," the I, Tonya star joked. "It was the cutest."

"She was like in these cutest little things you'd put a toddler in to go to bed," Margot added, "and then she was like rousing at us for not knowing the rules."

Barbie actress Emma Mackey previously shared another detail about the cast gathering, telling Empire last July that one of the activities involved table tennis.

"I don't play games usually, because I get so competitive and angry," Emma admitted. "But Scott [Evans] and I were top of the game."

Before Barbie premieres July 21, get a peek at the movie below.

(E! News and The Kelly Clarkson Show are part of the NBCU family.)

Co-stars Margot and Ryan Gosling sported matching rollerblades and coordinating outfits on the set of the Barbie movie on June 27.

Robbie and Gosling, who star as Barbie and Ken, share a laugh while filming by the beach in Southern California.