As a writer, it is easy to get caught up in the development of the main character and spend little time on the rest of the cast. But . . . don't! The plot will fall apart without strong supporting characters. Here are three pitfalls to avoid when creating supporting characters:
Furniture: Sometimes people tend to think supporting characters are kind of like furniture or wallpaper - just someone else to throw in so the plot doesn't look empty. Um, NO!
Supporting characters draw out the main character's strengths, weaknesses, and personality traits through dialogue and action. They help us understand what the main character is doing or saying and why. Supporting characters can be used as mirrors or yardsticks or interviewers or manipulators - all so the reader can understand and support or empathize with the main character.
Supporting characters also help move the action along. They do things or say things that force the main character to make decisions or to act, thus moving the plot forward. Even Tom Hanks in Castaway had a strong supporting character. Wilson. He seamlessly shows us how truly insane the isolation of the island has made Tom Hanks - all without speaking once!
Okay, so maybe you know the supporting characters aren't furniture; that they actually have jobs to perform. But while you're twisting them into your plot, make sure to avoid these two other common pitfalls:
Cardboard cut-outs: You know what I'm talking about. Those characters that seem to have been thrown in with some unique trait just to show diversity: a disability, a sexual preference, a socioeconomic background different from everyone else - these things are awesome, if your character has other traits too and you weave it all together to make layers in the plot and dialogue. But if that is all you've given that character, he will simply look like a one-dimensional cardboard cut-out. BORING and worse, FAKE.
Skydivers: The other thing that happens way too often is the appearance of a character just to serve a purpose. These characters drop in from the sky all deus-ex-machina-like to help the hero out or to wreak havoc - then they disappear. Never to be heard from again. I hate that. Nothing worse than some random character showing up in the nick of time. If the role is important, then make that character important. Introduce him and make his life count long before you need him.
So, yeah. Even though supporting characters are often minor and exist only to move the plot or define the main character, they are still characters. They have to be as three-dimensional as the main characters. No, the reader doesn't have to know everything about those characters, but the author does. To create real dialogue, real actions, and real reactions, the author has to know all that the characters have been through or are going through or will go through. That is what creates conflict and flavor and, well, reality, as the story progresses.
That's my two cents on creating supporting characters.
Happy writing :)
a Fall 2012 debut from Houghton Mifflin Harcourt.