Primary characters are easy. We pick them, we write abut them, we plan their lives. Secondary characters are a bit more wily. Sometimes we know exactly who they are are what role they play. Other times, we have no idea who will be a walk on and who will be a star. How do you know who is a throwaway character and who will eventually become an important one?
Characters that are there for window dressing usually stay there. They serve a small, specific function and once they're done, they're done. They don't affect plot, so they can be as faceless as needed.
Sometimes a walk-on role develops into someone a lot more important, and suddenly they have their own story arc. A throwaway character in The Shifter ended up being a major character, and it was purely by accident. I needed a walk-on and they were never meant to be more than that. But later, when I needed a character to play a supporting role, it just hit me that using the same one really tied the story together in a way I'd hadn't thought about before.
There was a reason this worked out so well.
This particular throwaway character was in a role that connected to the larger plot. I couldn't have used any other throwaway character in the entire book and still accomplished the same thing. (Which is a really good indication of a potential star hiding in the throwaways).
When you know you're going to create a character with more staying power, you can look back at some of your walk-ons and see if any would work. Not only does this help keep the number of characters down, but it allows you to deepen your story links and tie things together in ways that enhance your theme or even increase your stakes.
- Is there a throwaway in a position to make your protagonist's life harder?
- Support them when no one else can?
- Complicate their problems?
The flip side of this, is a character who is fun, but really doesn't do anything to help the story. These guys can easily take over and demand more page time, and you sometimes end up creating subplots for them. Be wary of these prima donnas. They seem really nice, so you'll want to help them grow, but keep an eye out so they don't start moving in without permission.
Promote the stars. Keep the spotlight hogs in the dark.