Pixar chose to portray the film's main character as a musician, because they wanted a "profession the audience could root for," and settled for a musician after trying for a scientist, which "[didn't feel] so naturally pure as a musician's life." Pete Docter described Soul as "an exploration of: Where should your focus be? What are the things that, at the end of the day, are really going to be the important things that you look back on and go, 'I spent a worthy amount of my limited time on Earth worrying or focused on that'?"
Animators used footage of several music performers, including jazz composer Jon Batiste, performing as reference for the film's musical sequences. By capturing MIDI data from the sessions, animators were able to retrace the exact key being played on the piano with each note and create the performances authentically.
In an initial draft of the story, 22 was the main character rather than Joe. 22 had a strong dislike for Earth, and the film took place entirely in the soul world. Joe was created to help show her why Earth is worth living in.
John Ratzenberger, who has voiced a character in every other Pixar movie, is not credited in this film. However, Pete Docter revealed that he does cameo in the film: his likeness is used for a character in the subway.
(at around 20 mins) The first soul assigned is number '108,210,121,415.' This lines up with the current (as of release) estimate from the Population Reference Bureau (PRB), which estimates that more than 108 billion humans have existed on earth (108 thousands of millions using the European system of counting).