The 45-year-old plays a highly-sexed blonde who falls in love with a convict in the new thriller, and director Lee Daniels said he asked Kidman to gain weight for the role so her character would appear more flawed, reported Contactmusic.
"It was important that she put on some weight. I said, 'I want you fat. I want your butt jiggling, I want your thighs moving.' She put on 15 pounds," he said.
Daniels also forced Kidman to tame her own locks and apply her own makeup in order to look less polished.
"I told Nicole, 'Look, I can't get some Hollywood make-up artist to do their Hollywood take on it. You have to do this yourself.' I thought she was gonna quit because she was so shocked."