The best advice I’ve heard about side characters came from an episode of the writing excuses podcast. It might have been this episode, but I don’t remember (they’re all great though). The advice is, “side characters don’t think they are side characters.”
You are a side character in someone else’s story. But, do you think about yourself that way?
Of course not.
You have a past, memories, emotions, dreams, family, friends, 401k. You are a well rounded human being. But you also get mad when the soda machine gives you diet and get excited when your favorite band releases a new song. Your side characters need to be that too.
The trick is, they’re not going to get as much page-time as your main character, so you have to be more subtle about rounding them out. Give us the real important things that make a person unique (how they handle stress or conflict, how they react to praise, etc).
Another way to make your side characters interesting is to give them goals (I think I learned this from a Scriptnotes episode). Their goals won’t be as lofty as the main character’s goal, but there are different levels of goal achievement here.
The side character who gets the most page-time will have a more developed goal-failure-success arc than someone who only has a few lines.
That being said, the character with a few lines can state their want at the beginning of the story and then have it paid off at the very end. Those kinds of payoffs are much easier and help your audience relate to those characters.
Jump. Build, Fly.
F.C. Shultz