Kidman is no stranger to the musical genre, having starring in 2001's "Moulin Rouge." It was director Rob Marshall who lured her back to work with the prospect of working in this all-star ensemble.
"I always get nervous because it's not really the thing that I do. It's a secondary thing and it doesn't come naturally to me, I have to learn the song and it's not like I can play an instrument or anything," Kidman said about singing on film.
Singing on-screen was something new for Marion Cotillard. She won an Oscar for playing famed singer Edith Piaf in "La Vie En Rose," but she lip synched in the 2007 film.
This time around, it's all her.
"I've always loved to sing. I've always wanted to do a musical and especially an American musical without knowing it would be possible one day," said Cotillard. "Singing is so -- it brings you emotions that are bigger than life."
Two-time Oscar winner Daniel Day Lewis also sings in the film. He said it was all about finding that invisible line between speaking and singing.
"Everyone loves to sing even if they can't sing, no one really knows whether they can or not until somebody says you can't sing, don't, stop it," said Day Lewis.
"Nine" is rated PG-13.