In the film, the "Big Little Lies" star plays an undercover detective, who infiltrates a California gang, and Nicole reveals she had to go to a dark place to get into character and the heaviness of the film began to weigh on her.
"I was very, deeply depressed," she tells Variety. "It was the point where my husband was like, 'When the hell is this going to end?'."
She also reveals that her man, Keith Urban, was "weirded out" when he saw the film for the first time: "Wouldn't you be weirded out if you were married to one person, and they show up looking like that?" she adds.
However, Kidman knew from the start it was a great role for her, because she sobbed after her first script read-through.
"They were considering someone else for the role," she says. "Then the other actress didn't want to do it. I was like, 'I want to'."
Director Karyn Kusama also knew Nicole was the perfect fit for the part.
"She talked a lot about shame, what it does to your body and mind," Kusama says. "I realised a couple of things. One, she was going to fully embody the character in another physicality. And I knew she was going to bring a humanity to the character."
For the role, Nicole also developed a limp and wore prosthetics on her face. She also trained with a military veteran in her adopted Tennessee to learn how to use guns and assault rifles.
"All of the weapons, I can load them, I can fire them, I know them," she says. "I trained for about a month, but every day. It was cold, and I was tired, and I would just train. And my hands cramped, because I don't have big hands and I don't have strong hands. It was awful."