Character Error When Samantha is leading Theodore to the pizza stand at the carnival, she tells him to turn around by saying, "Do a 360." If he did that he'd be facing the same way he was already going; she should have said, "Do a 180."
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.
Most of the city backgrounds, especially the ones featuring skyscrapers, are actually filmed in Shanghai. You can see many very identifiable skyscrapers such as the Shanghai World Financial Center. There are even Chinese signs if you look carefully. Apparently, Shanghai now is what Los Angeles will look like in the future.
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.
Chosen by The Film Society of Lincoln Center as the featured film to close its prestigious New York Film Festival on October 13, 2013, in NYC.
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.
During some of the more emotional scenes, Amy Adams would sing songs from famous musicals like "Annie" and "The Rocky Horror Picture Show" in order to cheer herself up. Eventually, Joaquin Phoenix would join in and they would sing together. However, they stopped after they saw Spike Jonze filming them.
The first time Portia Doubleday and Rooney Mara are in the same movie since Youth in Revolt (2009).
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