Supporting characters are as vital as their name implies: they support your main characters and plot arcs so that everything runs smoothly (or horribly, as the case may be--disasters make good fiction). Done right, supporting characters make your protagonist look good (or bad, as the case may be) and offer a much-needed alternate perspective. This golden principle will make sure all of your supporting characters are being used effectively!
Revise It! The Miniseries: Six Factors to Unpacking Your First Draft tackles six key areas in content revisions to get your first draft into great condition!