"I'm so excited to be interviewing you today," Witherspoon said to Kidman.
The two actresses began by discussing the basics: where they were born.
"Well, I was born in 1976 in New Orleans, Louisiana," Witherspoon told Kidman.
Kidman then surprised Witherspoon by sharing her Hawaiian heritage, "and I have an Hawaiian name, Hokulani," Kidman said.
"Let's get to the important stuff," Witherspoon said. "What was your first reaction when you read the book, 'Big Little Lies?'"
Kidman said she was "riveted" and that she read the book in around "48 hours."
During the interview, Witherspoon admitted that she had always looked up to Kidman.
"We didn't know each other very well before we started but I just always ... I always looked up to you. You're such an incredible actress," Witherspoon said.
Kidman replied, "I call myself your big sister now."
"I'd never had the experience of working with another woman so closely that I got to call you and say, 'What do you think I should do with this character? I don't like this scene and I don't know how to play this,'" Kidman said.
When asked by Witherspoon what her favorite scene to film together was, Kidman said: "I love the scene in the car where we talk about life, where we talk about the desires that we have.
"How we don't feel comfortable just being a mom, that there's secret hidden things that we want," Kidman said. "I think that's an interesting truth for a lot of women, because we love being mothers but a lot of us have creative juices as we'll that we're going, 'Well, how does ... it come out?' Sometimes it comes out by being a great cook, sometimes it comes out because you can write or you can paint. But certain women don't have that outlet. Where is it? And we all need it, you know?"
Witherspoon added that one of her favorite aspects of "Big Little Lies" was that, "it was a huge opportunity, with five distinct female parts, where women are talking to each other how they really talk to each other, about issues that are really affecting their lives every day ... and that's so important, that female friendship. And those bonds are what, I mean, my girlfriends are my life."
"And as much as there's the entertainment part of it, there's also this camaraderie," Witherspoon said of the series. "You see what we do for each other as friends, and what we do for each other behind the scenes in terms of protecting each other, holding each other's secrets."
"They always say there's a boys' club," Witherspoon said, adding that they made it a "girls' club."
Kidman agreed, saying, "there's a girls' club."
"Big Little Lies" premieres Sunday, Feb. 19, 2017 on HBO.