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Most stories have other characters besides the hero and the antagonist. These secondary characters may be part of the main storyline or they may play a role in a subplot. As interesting as these characters and their stories may be in themselves, their importance lies in how they affect the protagonist.
We all interact differently with different people in our lives. You might be lighthearted with your brother, feisty with your boss, and downright homicidal with your in-laws. Just as other people bring out different sides of your personality, supporting characters bring out different sides of the main characters. Someone has to talk sense into a stubborn protagonist, melt the heart of a tough guy, or rile the feathers of a hothead. We need other characters to push their buttons because that's what creates conflict. Will the protagonist listen to advice, let go of his fear, and embrace love? Or will he resist, fight, and end up alone? Will he overcome his flaw? Or will his flaw get the better of him? Supporting characters allow the story to ask and answer these questions.
If your heroine works double shifts to make ends meet, her boss is important so long as he either hinders or helps her cause. Let's say he keeps hitting on her until she tells him off and gets fired. Then the boss will actually have played a role in her storyline. If, however, the only reason he's there is because employees have bosses and the screenwriter thought it would add some realism to have her chit-chat about the weather with him, then he's dead weight. He should be cut. Supporting characters are meant to feed the main storyline.
The presence of secondary characters should feel organic to the whole. To be believable, they need to come across as characters in their own right without overshadowing the main characters. The more important a supporting character is, the more fully developed he will be. A major secondary character will not only have his own agenda and defining traits, but possibly his own character arc, too.