Any discussion about the importance of minor characters to a novel, film or play, begins with a discussion about characters in general. Characters -- the people that inhabit the story -- must be well-rounded, realistic and should resonate with the reader. The reader should connect with all the characters, heroes, villains and bit players. After all, great characterization turns a novel into a new world. And in that new world, minor characters play a major role.
As far back as Ancient Greek theater, minor characters have served the story by delivering exposition, or vital information necessary to move the story forward. For example, in "Oedipus the King," by Sophocles, it is the chorus that helps propel the story forward and provide information to fill in backstory. This does not come from Oedipus, the main characters. In "Star Wars," Luke Skywalker's aunt and uncle provide information about Obi-Wan Kenobi and give us our first glimpse into Luke's secret backstory.
Adding Dimension to Leads
Just as in real life, the people that surround the main character speaks a lot about the protagonist's personality. In Michael Chabon's "Mysteries of Pittsburgh," the main character, Art Bechstein, is trying to move away from his father's ties to organized crime. But he quickly makes friends with a young man, Cleveland Arning, who is every day delving deeper into organized crime. Arning's connection to the mob tells us that Bechstein is more like his father than he wants to be, as he is attracted to the same people and lifestyle as his father.
Sometimes minor characters act symbolically and help propel the story that way. A great example is the "Girl in Red" from Steven Spielberg's "Schindler's List." The film, which explores the true story of Oskar Schindler's rescue of hundreds of Jews from the Nazis, was shot almost entirely in black and white. During the film, Schindler, resolved to ignore the Jews' plight, sees a young Jewish girl walking amongst a Nazi siege. The girl appears in a red coat -- the only color in the film -- and Schindler sees her. The girl, who does not appear again, symbolizes Schindler's sudden realization that he must do something to stop this horror.
Quite often, minor characters act as comic relief, especially in dramas and action films. Where the main character wouldn't act bumbling or scared or goofy, minor characters can. Gabby Hayes, comic star of dozens of Westerns, is a good example. But you can easily find them in today's TV shows and films: Jon Turturo's character in "Transformers," Jamie Kennedy's character in the "Scream" franchise and Jorge Garcia's character in "Lost."