Following the footsteps of Rene Zellweger who gained oodles of weight for role in Bridget Jones, the svelte star has already gained 10 pounds.
A source close to the actress said that the role is one of the most challenging roles that Kidman has been offered till now.
"Nicole has said this is one of the most challenging film roles of her career," the Daily Mail quoted a close friend of the actress, as saying.
"She is naturally very slim and loves to run and swim so she is always very toned. The part she plays demands that she gets fatter throughout the movie as her character slips into middle age.
"It's a very demanding role because there are a lot of explicit lovemaking scenes. The film is set over several decades so her character changes a lot. Nicole was originally planning to use prosthetics but was told by the director, Stephen Daldry, that it would be more realistic if she gained the weight.
"She has been eating about four meals a day. It's not usual to see her work her way through a big plate of pasta, with bread, and then a dessert," the friend added.
For gaining pounds, the actress has been gorging on her favourite puddings, the pal reveals.
"She's having to gorge herself. She has quite enjoyed gaining the first 10lb and has been having lots of her favourite puddings as well as chocolate," the friend said.
"Nicole loves her new bosom, but she is finding eating so much makes her lethargic and sleepy. She's usually running on adrenaline and has to be reminded to eat a meal. Now she's eating four, and sometimes five small meals a day,' the friend added.
The Oscar winner will play Hanna Schmitz in the film about the emotional repercussions of the Holocaust.