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.
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.
Rooney Mara and Joaquin Phoenix who play a married/divorced couple in the movie later married and had two children together.
The apartment scenes have all been filmed on location. In order to get the best lighting conditions possible Dutch cinematographer Hoyte van Hoytema decided to change all windows surrounding the apartment on the 34th floor. Additionally he decided to install large mirrors on the helicopter platform of the skyscraper across the street. This gave him the possibility to bounce back sunlight into the apartment.
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.