Witherspoon might not know Nicole Kidman as well as she thinks.
In keeping with the theme of their new HBO show, ET got the cast of Big Little Lies to take a quiz to see how much they really know about their co-stars.
ET's Nichelle Turner caught up with the ladies at the miniseries' Los Angeles premiere, where executive producer and star Reese Witherspoon was the first to test her knowledge of real-life friend and co-star Nicole Kidman.
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"I'm going to say one thing about her that's true and one thing that's a lie. You tell me which one you think is true and which one you think is a lie," Turner explained. "Nicole's high school nickname was 'Legs,' and she is deathly afraid of butterflies."
"She's not afraid of butterflies," Witherspoon said with a laugh. "I agree that she was probably called 'Legs.' Am I wrong?"
"She suffers from lepidopterophobia -- a fear of butterflies," Turner answered. "And her high school name was 'Stalky.'"
It appears as if Witherspoon might not know Kidman as well as she thinks, which is the premise behind her highly anticipated new series.
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Based on Liane Moriarty's best-selling 2014 novel of the same name, Big Little Lies centers on a group of women -- all in different stages of motherhood -- in Northern California, whose seemingly perfect lives begin to unravel to the point of murder.
The seven-part miniseries also stars Shailene Woodley as the single mom who's new in town, and Zoe Kravitz, who plays the new wife of Witherspoon's ex-husband.
Kravitz did a little better with our quiz than Witherspoon when we asked her about Woodley.
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"Which one is the lie?" Turner asked. "Shailene Woodley is a pack rat. Shailene Woodley once lied to her agent and said she was going camping in New Zealand, only to stay home in L.A. for two months without being bothered."
"That sounds true," Kravitz said. "That so sounds like something she would do."
"You are correct!" Turner said. "That is absolutely true."
Big Little Lies premieres Sunday on HBO.