Mac is not the first character I created, but she has been with me over half my life. Maybe that's why she's my favorite. Or maybe it's because I know her so well. After all these years, I've learned a lot about her childhood, about what makes her the person she is when the story begins. She's almost as real to me as I am.
Generally speaking, characters come in various 'flavors': Main characters, secondary characters, background characters and spear-chuckers, which some people call walk-ons. A good author knows what type each of his/her characters is, and devotes the correct amount of time to discovering their background.
Spear-chuckers/walk-ons are there only to do one thing; sell a movie ticket, deliver a meal or drink, or toss spears to ward off evil fire-breathing dragons (like a jet plane or spaceship shuttle). Probably the most you need to know about them is a bit of their culture (if it's different from the culture of your main characters) and whether or not they're having a bad day. A waitress having a bad day might dump a trayful of food on a main character, which can lead to interesting consequences.
Background characters need to be known a little better. Perhaps something they say to a main character, based on the personal history of the background character, will make the main character shift their thinking. So you need to know a few basics about background characters, maybe where they grew up; are they part of a large family or an only child; a poor family or wealthy?
Think about the walk-ons and background characters in your life; the post man, bus driver, store clerk, auto mechanic. How much do you know about them? You should know at least that much about the backgrounds of your 'less seen' characters in your stories. In my opinion, walk-ons can be pretty cardboard. Background characters need a bit more depth.
To be continued…