The primary characters in a work of fiction are fairly easy to keep track of. You need a protagonist, or hero, and an antagonist or villain of some kind. If you're writing a romance, you need a love interest.
And . . . we're done. For the average short story or work of flash fiction, you don't have space to add more characters than that. You'll be lucky to develop just two in the span of a few hundred or a thousand words.
But for longer short stories, novellas, and novels, and especially for series, it's sometimes important to add secondary characters to the work.
One of the quickest ways to learn about the different roles secondary characters can fulfill in your fiction is to study movies, because writing effective secondary characters is the third thing movies teach us about writing fiction.
Here are some of the roles secondary characters can fulfill:
1. Secondary characters act as sidekicks for the protagonist.
Think Sid the Sloth in the movie Ice Age, or Donkey in the movie Shrek. Manfred (Manny) the mammoth in Ice Age is on a mission to be alone so he can grieve his recent losses, and he and Sid fall in together by circumstance.
By the end of the movie, Sid is just as important a character as Manny, and you've learned a lot about them through their interactions together. Sid encourages Manny to break out of his sadness and commit to taking care of an abandoned human child until they can return the child to its "herd," a move that reunites a family and brings healing to Manny's heart.
2. Secondary characters act as foils for the protagonist.
A foil is a character who is the opposite of the protagonist in pretty much every way. The foil therefore showcases all of the protagonist's unique qualities by drawing attention to the way the two of them are so different.
In the movie Princess Diaries with Anne Hathaway as the totally inept and klutzy (at first) Mia, her foil (and antagonist) is the blonde cheerleader, Lana. While Mia can't even cross her legs without falling off her chair, Lana trots around in high heels and manages to look as sleek and sophisticated as a European model. Her grace emphasizes Mia's awkwardness, which is why the transformation Mia makes from awkward to chic has an even greater impact.
3. Secondary characters act as mentors for the protagonist.
A mentor is a character who's been before where the protagonist is now. Gandalf, the wizard in the movie The Hobbit is a mentor to Bilbo. The story is about Bilbo's adventure and transformation, but Gandalf supports him through with bits of timely wisdom and encouragement.
Some secondary characters fulfill more than one role. Because they are more well-rounded, the story is richer and more believable.
For instance, in the movie One for the Money, based on Janet Evanovich's Stephanie Plum series, Lula is not only Stephanie's sidekick but also a foil for her (Lula's African-American, likes to dress in tight spandex, and used to be a hooker; Stephanie is all-American white, wears jeans and t-shirts, and used to sell lingerie).
On the other hand, Ranger is both foil (he's Cuban-American, wears all black, is ex-military, and is the best bounty hunter in the company, while Stephanie isn't, doesn't, isn't, and is the worst bounty hunter in the company, in that order) and mentor (he takes Stephanie under his wing and tries to teach her how to be a more effective bounty hunter so she doesn't get herself or somebody else killed).
What secondary characters stick out in your head from movies you've seen or books you've read? What other roles can secondary characters fulfill? What roles do your secondary characters fulfill in your work-in-progress?