This is something different from character development. What I refer to here is the act of making certain each character is unique, but also that each character is described uniquely. So it’s more about what the author and narrator have done than what the character has done.
In my spreadsheets I like to make sure I describe what every character is wearing the first time we see them. What color is their hair? What marks them as different? Do they have unique mannerisms. I put all of that in my spreadsheet. When were they born? Who are they related to? What is their secret? They each have a dossier!
In my timeline notes I mark the major events of their lives. You don’t want to refer to John’s gender re-assignment surgery three years before he became Juanita!
The draft I am working on right now is partly about making sure every character gets an appropriate introduction and description.