Minors allowed!

For some thoughts on minor characters I would like to begin by quoting James Scott Bell*: "Supporting players should serve one of two purposes in a story. They either help or hinder the main character. They are allies or irritants."
Mr. Bell hit the nail on the head particularly referring to them as "supporting players". Think of the Academy Awards, affectionately called "the Oscars." There's a category for Best Supporting Actor (or more precisely, Best Actor in a Supporting Role). Let's take a look at last year's winner Heath Ledger for his role as the Joker in The Dark Knight. What would the story be without the Joker? Pretty boring, right? The Joker was an "irritant" (and that is an understatement). But then again consider the character of Harvey Dent played by Aaron Eckhart. Dent is a minor character who begins as an ally who helps and then becomes an irritant that hinders. (According to http://www.imdb.com/, the film grossed $533,316,061 so I'm taking a chance that you've seen this movie). Minor characters do not have to stay one way or the other.
What about Sam Gerard, played by Tommy Lee Jones (another Academy Award winner for Best Actor in a Supporting Role), the relentless U.S. marshal who pursues Dr. Richard Kimble in The Fugitive. Is he an ally or irritant? To Dr. Kimble, he is an irritant. Remember that the minor character must help or hinder the main character. That he is a law enforcement officer chasing an escaped criminal does not matter. The story is about a wrongfully accused man trying to find his wife's killer and prove his innocence.
Minor characters who are an ally can add something special to a story, too. Take Sam, the hobbit from The Lord of the Rings who stays with Frodo from the lush green Shire to the rocky fires of Mount Doom. In The Fellowship of the Ring, Frodo agrees to destroy the One Ring. By the end of The Return of the King, Sam is just as much as part of the journey as his master. Frodo would not have made it without Sam, who ends up literally carrying him.
And what would Casablanca be without Sam (Dooley Wilson)? Sam keeps Rick (Humphrey Bogart) in check. He was with Rick in Paris and he went with him to Casablanca. He remembers Ilsa, when the Nazis invaded Paris and when Rick stood in the rain reading the letter that Ilsa wrote telling him she would not be leaving with him. Sam is Rick's partner, confidant and friend. Without Sam, Rick's Cafe Americain would just be another saloon.
Have fun with your minor characters! To quote Mr. Bell one more time regarding allies and irritants: "If they aren't one or the other, what are they doing in the story except taking up space?"

*James Scott Bell, "Write Great Fiction: Revision & Self-Editing", Writer's Digest Books