According to Brian De Palma, the lead actresses arrived in Berlin a week before shooting started to rehearse the screenplay and had a lot of suggestions for the relationship of the characters, including making the lesbian subtext a more obvious element of the story. De Palma found himself overwhelmed with changing the screenplay so close to shooting, so he brought in Natalie Carter, co-writer of Crime d'amour (2010) by Alain Corneau, on which Passion (2012) is based on, and she re-wrote some scenes to accommodate Rapace and McAdams.
The trivia item below may give away important plot points.
Rachel McAdams said, "Playing Christine was a challenge. She's kind of wickedly delicious, but not too too much. Those characters can be more fun than the ingénue, the leading lady, where there's the expectation of having the audience like you. I saw the film not too long ago, and I found the sexuality quite restrained, even for Brian De Palma. I found it more about possession, and not about wanting to have sex with each other, and be physical, but wanting to possess the other. I think he's dealing a lot with vanity, and narcissism. You want the other person to reflect what you want to be. I felt like the only person I could do this with was Noomi. It certainly made it easier. I was nervous!" (Entertainment Weekly)