If you watched the original “Star Trek” TV show, Captain Kirk and the other main characters would always explore a dangerous planet in the company of one or two minor characters who we had never seen before. Inevitably, some horrible fate would befall this minor character, letting us know the danger the major characters were in.
That’s the purpose of minor characters, to show us the hero’s fate if he or she should fail.
In “The Art of Self Defense,” a meek accountant has been beaten up by strangers and thus motivated to take karate lessons. While attending the more vicious night class, he’s horrified to watch the instructor break the arm of another student.
In “Little Monsters,” a kindergarten teacher is trapped in a petting zoo during a zombie apocalypse. One of her assistants is a struggling musician who’s secretly in love with her. Another man trapped with her is a TV children’s celebrity actor. The struggling musician is the hero and is torn between being selfish (by focusing solely on winning the heart of the teacher) or being selfless (by focusing on helping the teacher protect the children from zombies).
To highlight the hero’s fate should he remain selfish, the TV celebrity actor tries to escape and selfishly leave without helping any of the children. Needless to say, he gets devoured by a zombie as punishment.
Minor characters exist solely to show the hero and the audience the hero’s fate if he or she should fail. So you need to know how your hero should fail and then show someone else failing in the hero’s place.
In bad movies, minor characters pop up and disappear for no reason. In good movies, minor characters pop up for a reason, and disappear only when they’ve served their purpose of foreshadowing the hero’s fate should he or she fail.
Keep you minor characters to a minimum, but use them to dramatically show the danger facing the hero. The hero always needs to be in danger, so having a minor character fail is the best way to showcase the possibility of failure.