So here's me, throwing out some advice and seeing whether I want to take it myself. As always, do what works; hopefully, this helps.
The standard piece of advice for writing a minor character goes like this: add something unexpected. It's that kind of advice that really gets under my skin. It's useful advice, but only if you already know how to add something unexpected, and if you knew how to do that, you wouldn't need the advice in the first place. I mean, how do you know what's unexpected? And why bother adding it in the first place? And when should you add it? Can you build plot points off of insignificant details like that? Should you?
Two ways that I've found to approach the problem:
1) Simple & effective. Imagine a range with "random" on one side and "orderly" on the other. A random number doesn't convey any useful information; a perfectly orderly number (e.g., 0000000000000...) doesn't either. Add details about your minor characters that aren't random but aren't totally in keeping with expectations, either. The essence of the detail is that it has to "fit" with the character.
Example: A modern-day witch.
Orderly: A sexy, modern-day witch with brunette hair.
Random: A sexy, modern-day witch with purple hair who does things exactly as the witch with brunette hair.
Middle: A modern-day witch with purple hair who acts like a person with purple hair might be expected to act, sullen and alienated, with a geeky fangirl love of Neil Gaiman.
Pros: Simple. Acknowledges that people have lives that don't necessarily fit in with our prejudices. Gives story texture.
Cons: Too many charming but essentially meaningless minor characters gets distracting. It's hard to make this work consistently.
2) Complex & resonant. Figure out what point of the story is, and then make the character fit the purpose of the story as well as her minor function in the plot.
Pros: Gives the story more integrity and more opportunities to use themes, subplots, etc.
Cons: It makes my brain tired.
First, ask "What's the purpose of the character?" I've come up with four, so far. Each purpose doesn't stand alone; minor characters, like anything in a good story, should serve more than one purpose. Minor characters should:
- Perform an action that move the plot ahead.
- Allow the main characters to show off or display their character traits/conflicts.
- Establish setting.
- Symbolize an element in the story.
Main Character Development. The little girl with a kitten up a tree; she begs the heroine to save the cat. The heroine is afraid of heights. What will the heroine do???
Establish Setting. The long-winded professor whose biology lectures are so dull the heronie is teased into responding to her best friend's notes about the romantic lead during class.
Symbolize Story Element. The drunk driver who hits the little kitten-girl, symbolizing the dark side of chance (chance brought the romantic interest into the heroine's life; chance could take him out).