The 43-year-old and Aaron Eckhart play a couple struggling with loss in their new flick Rabbit Hole.
Kidman, who is mother to Sunday Rose with husband Keith Urban, said it was not hard finding the
kind of emotion that was needed for this movie.
She told ETonline.com: 'As soon as you're a mother, it's there; it's not hard to find.
'All of the emotions were like quicksilver, they were so quick to come and I had them there at my fingertips almost all the time.'
Nicole added that maintaining the grief-stricken character's state of mind was the most difficult part of making the film.
She said: 'I'm offering up a deep truth, because I didn't want to fake any of it. You don't want to go crazy, so it's kind of like existing in a limbo.'
Rabbit Hole hits cinemas in the US on December 17.