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 cannot see or hear her.
Mark Waters chose to use a real cable car, instead of a prop cable car. So when the realtor follows David across the street to Elizabeth's apartment, she runs across the tracks in her heels to avoid the cable car.
David and Elizabeth's apartment building is really about six stories tall, but the view from the roof (done with CGI) is one from a much taller building. They look down upon other buildings that are of a similar height.
Two actors from this film also starred in Marvel films: Mark Ruffalo starred as Bruce Banner, a.k.a. the Incredible Hulk, in the Marvel Cinematic Universe. Additionally, Donal Logue starred as "Quinn" in Blade (1998), and "Mack" in Ghost Rider (2007).
The trivia item below may give away important plot points.
When Elizabeth and David first visit the comatose Elizabeth, Reese Witherspoon, most of the time, played Elizabeth "the ghost", and her body double (who looks so like Witherspoon, that some people had mistaken her for Reese Witherspoon) was on the bed.