In the middle of March 2009, Liam Neeson interrupted filming his scenes, in order to visit his wife Natasha Richardson in the hospital, after she had a skiing accident. The brain injury she received from this accident, lead to her death a few days later. Just few days after her death, Neeson voluntarily returned to the set, and completed his performance. The filmmakers changed the script accordingly, and Neeson completed his performance in two days.
Mirrors are frequently featured in the film, and when they are, there is usually more than one mirror in the same shot. A prime example would be the scene in the hotel room between Chloe and Catherine.
The trivia item below may give away important plot points.
Amanda Seyfried and Julianne Moore recorded a voice-over that would have played over the final scene. Both gave more insight into the reason for Chloe's suicide from the speaker's perspective, but in the end, neither of them was used. Both endings can be seen on the bonus features on the DVD.