Continuity When Theodore is walking the corridor towards Amy's apartment, the exit signs have Chinese writing above English, showing that the scene was clearly shot in China and not Los Angeles, where it takes place.
Factual Mistake On the spine of the physics book he was reading, "Unknown" was misspelled as "Unkown".
Samantha is the name of a text-to-speech setting on Macintosh computers that closely resembles the voice of pre-iOS7 Siri, Apple's virtual assistant for iPhone. It's more than likely that the name of OS1's virtual assistant being Samantha as well is a reference to this.
One legacy of Samantha Morton's casting is the name of Theodore's operating system. Both lead female roles take the same name as their lead actresses, Amy from actress Amy Adams and Samantha from Morton, but since she had to be re-casted at the last minute, the OS's name stayed Samantha.
Samantha Morton was originally the voice of Samantha. She was present on the set with Joaquin Phoenix every day. After the filming wrapped and Spike Jonze started editing the movie, he felt like something was not right. With Morton's blessing, he decided to recast the role and Scarlett Johansson was brought and replaced Morton, re-recording all the dialogue..
Jonze's friend Steven Soderbergh helped him trim the film by cutting down an early version of it from two-and-a-half hours to 90 minutes...all in twenty-four hours' time.
This is the only movie Spike Jonze has written as well as directed, completely from his own original script.
Amy Adams said writer/director Spike Jonze would essentially lock her and Joaquin Phoenix in a room together for an hour or two every other day, and make them talk to each other. Jonze did this so that the actors could get to know each other better. Adams credits this for her and Phoenix's close friendship.
Carey Mulligan was originally cast but dropped out due to scheduling conflicts. She was replaced by Rooney Mara.
Chris Cooper filmed a few scenes but his role ended up completely cut from the final movie.
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