Watts told reporters in New York recently she initially hesitated to take a role in the thriller because she was pregnant when given the screenplay.
"It came to me that Clive Owen was doing a movie with Tom Tykwer and that's what made me instantly want to read the script, despite the fact I felt there was no way I'd be working that close to having my first child," she said.
"Then I met with Tom and the way he talked about the film, his sensibilities, cinematically, it sounded very interesting to me. The subject also played a part and Tom also said, 'Let me go away and see how I can work the schedule and make it doable for you.'"
She said the solution was to tighten the filming of her part to 21 days over five consecutive weeks, and have the rest of the cast and crew start two months before she arrived on set when her son was 3 months old and she "felt ready to go back to work."
Watts went on to say she doesn't think she could have done an intense movie in which she was the main protagonist at that point in time, however.
"I couldn't have played Clive's part. Some of the films I've done, the characters are in a very emotional or turbulent state and Eleanor Whitman was actually quite together and I felt I could handle her," said Watts, who has since given birth to her second son.