Here’s a small portion of my protagonist’s biography, which I put down as I wrote the first book featuring her:
Veronica Leigh Slate was born May 5, 1955, in Roanoke, Virginia to Archibald Kingston Slate and Elizabeth Leigh Slate. They lived on Day Avenue. Veronica's parents moved to Arlington, Virginia in August 1962 when she was seven years old because her father began working for the FBI, specializing in anti-terrorist work. In 1973, she had just graduated from high school in Arlington. About six weeks after Veronica's 18th birthday, her parents had insisted she go with them on their vacation trip to the Pocono Mountains of Pennsylvania. As usual, they stayed at a grand old resort hotel called Pocono Manor Inn, a vine-covered castle that managed to be both inviting and forbidding.
In June 1973, Veronica met LTJG Sam Treace, four years older than her. He was the most handsome man she'd ever seen. He became her first husband.
As I wrote more books about her, I created more background. Her biography is now 50 single-spaced pages. It’s in narrative and in detail because Veronica has appeared in three published books by that time.
Probably 75% of this material did not appear in any of the books; it was for me, so I could know her better. You don't need to create a 50-page bio before you begin writing a book, but at least create a shorter one, confident you can add to it as you write the book and learn more about your protagonist.
Keep track of supporting players.
For supporting characters, you just need to keep track of what you have written about them so you don’t forget who likes Corona, and who was scared by a bear when she was three. Just remember whatever you've written about the supporting characters so you don't get it wrong in the future. You need only invent enough background for them to suit the needs of the novel.
No matter the genre, the stories we write are about people and events in their lives. Readers identify with characters because of their parallels with themselves.
----- (From NOVEL SECRETS available in paperback and Kindle form.)