Your main characters don't appear on the pages of your novel alone. They are surrounded by and, in most cases, vastly outnumbered by your supporting characters. As the name indicates, they offer your story and your protagonists and/or antagonists support. Their development is as critical as your main characters'. Here are the four primary roles of supporting characters in most works of fiction:
Establishing setting: Setting isn't just landscape and architecture. Supporting characters are just as crucial to setting. Accents, dialects, attitudes, cultural norms, etc., are just a few details that supporting characters can lend to a story's setting.
Acting as comedic vehicle or voice of reason: Supporting characters can give a story balance. If you're writing an intense thriller or mystery, a supporting character can provide a handful of laughs to allow the reader to breathe. If you're penning a novel where your main character is on a journey of self-discovery, supporting characters can show him or her the way.
Adding a curve or two to your twist: Sometimes authors use supporting characters as a diversion. What is a seemingly innocuous supporting character may actually either be the springboard to your main plot twist or he or she may be the actual twist.
Contributing a piece of the puzzle: Why is your main character a steely eyed tough guy or a sharp-witted policewoman with finely honed investigative skills? Such people aren't born, they are made, and they are made by the people in their lives--supporting characters.
As you develop your supporting characters, concentrate on what purpose they serve. If they don't meet the criteria of any of the above roles, there's a better-than-good chance they are weighing your story down and can be trimmed during rewrites.
Richard Ridley is an award-winning author and paid CreateSpace contributor.