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.
(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).
The "Production Babies" part of the credits is retitled "Recent You Seminar Graduates." Before the end logos roll, it says, in addition to the usual "Created by Pixar" text: "...and in homes at least six feet away from each other in the Bay Area," referencing the fact the animators worked at home due to the COVID-19 Pandemic.