The "Big Little Lies" star plays a broken, damaged police detective in the gritty crime thriller, which follows her character as she tracks down gang members connected with her previously-botched undercover operation.
Nicole underwent a complete image transformation and appeared almost unrecognisable with her short, dark hair and haggard appearance, and when her girls Faith, seven, and Sunday, 10, caught sight of mum in her movie make-up and wig, they came up with a new nickname.
"They called me 'granny,'" the Australian actress told the Los Angeles Times. "They're like, 'You're our granny now.'"
Tickled by their reactions, Nicole decided to play along and jokingly told them she was going to show up at their school for grandparents' day, and pose as their grandmother.
Nicole Kidman almost gave up acting. This month, she's staring in three movies https://t.co/DAgMJLspyf
— Los Angeles Times (@latimes) 14 December 2018
"I thought it'd be kind of quirky and funny and make for a good story for when they were older," she explained.
But Sunday was not impressed and quickly urged Nicole not to embarrass her like that.
"She's just like, 'Whatever you do, never, ever do that,'" the star chuckled. "So I won't be dressing up as their granny - even though that's what they called me!"
Although Nicole was happy to joke around with her character's appearance, she recently admitted it was a tough role to shake off at the end of each work day, and her husband, country singer Keith Urban, couldn't wait for filming to wrap.
"My husband was like, 'When is this going to end?' As an actor, you take on things and some you can shed and walk away from and others you can't," she explained on Britain's "The Graham Norton Show". "The character had to seep out of my pores and I didn't know how to perform that, so I just had to live it. I was a bit of a nightmare!"
Nicole's dedication to the role is already paying off - it's earned her a Golden Globe nomination for best actress for the 2019 ceremony.