During some scenes and rehearsals, director Mark Waters had Mark Ruffalo wear an earpiece to receive his lines from Reese Witherspoon, who was not on the set, replicating the situation depicted in the film where David has to relay Elizabeth's messages to the others who can neither see nor hear her.
Co-screenwriter Leslie Dixon suggested shooting in the apartment where she used to live. She also suggested Jon Heder for the role of Darryl.
After receiving a proposal with a rough outline of the story, the producers bought the rights to the novel without having read the original French book.
According to the DVD commentary, Elizabeth's apartment rooftop was completely built on-stage, with CGI for the San Francisco skyline.
After a bad take while shooting, Mark Ruffalo jokingly jumped out of a window on the set. He landed on a catwalk just outside the window, but still scared the crew.