Creating minor characters in short stories and novels is often a mix of intuition and expedient puzzle construction. The plot tells you what you need the person to do whether s/he’s a maid or a butler or a car salesman, your intuition tells you about the character’s demeanor, and applying our jigsaw puzzle skills leads you to the traits (and backup research) to make the character real within the scope of his/her role.
Obviously, a character with a recurring presence throughout the novel needs more substance than a bartender who doesn’t even have a line of dialogue. It’s important, though, not to box yourself in because you may need that character later–that is s/he may slowly get a larger role in a novel or its sequels and/or may need to become more than s/he appeared to be when the reader first met them.
For example, my character Pollyanna appeared more or less as a walk-on character in book 3 (Lena) of the Florida Folk Magic Series, was enjoyable to work with as a writer, and also was a hit with readers. So, she became the main character in book 4 (Fate’s Arrows). Each time she appeared, I added depth to the character making sure the new information about her didn’t conflict with what I’d said before.
We learned over time that she was a marine nurse at MASH units in Korea and Okinawa. We learned that when marines took karate courses in Okinawa (where Shotokan karate was developed) Pollyanna tagged along and–to the surprise of the men–became quite good at it. I used legitimate Shotokan strikes in Fate’s Arrows and that meant watching a lot of video instruction about doing them correctly.
This is one example of taking a character and adding skills or traits that weren’t necessary when s/he first appeared. In Pollyanna’s case, she was always an enigma to those who knew her, an approach to life that would later fit with her CIA association. Had she been a conservationist, woodcraft skills that weren’t required in her first scene could have been added in subsequent stories because the stage would have been set for her to develop in that way. You could say the same for many avocations or careers, as long as you didn’t box yourself in when the character first walked into the story.
Or, if you plan everything, then you might have created that walk-on character one way or another, knowing that you were going to use him or her in multiple sequels. That’s too tedious for me, but a lot of writers like getting everything nailed down before they begin writing.
Now, in my novel-in-progress, Pollyanna is again the main character and I am building her style and way of life by introducing the reader to the concepts behind Karate that would have been overkill in her first or second appearance. She has a Zen approach to life. That fits with Karate. So now I can add the concept of Bushido (a moral code) as well as the general precepts of Shotokan developed by Gichin Funakoshi that stress a unity of mind and body. That Pollyanna adheres to these approaches to life, not only explains why she does what she does, but makes her come across as a character of a lot of depth.
Had she been a conservationist, I might now be adding in such concepts as a forest as a unit rather than isolated trees and how that impacts the environment, forest management, and the careful use of fire. I probably would have followed the concepts and researchers behind Richard Powers’ novel The Overstory and made them (like Bushido in Karate) an important philosophy of the character.
Needless to say, every “walk-on character won’t end up becoming the protagonist in subsequent novels. But it’s fun when this happens because, for the author, they are rather like a child who likes playing with blocks who ends up becoming an architect with, say, the viewpoint of (possibly) Frank Lloyd Wright and his Prairie Style and its influences from the natural world. Such influences might even become a part of the character’s developing persona.
When characters are involved, it’s easy for an author to feel like a parent.