Recently, I saw the following quote by Jocelyn Hughes on Facebook, Treat all your secondary characters like they think the book’s about them. It made me think about the roles that secondary characters play, and why they’re important:

 Everyone needs a best friend/sidekick to laugh and cry with!
Secondary characters provide a barrier to what the main character wants, or helps them achieve what they want.
Secondary characters show up in so many roles; best friend, love interest, killer, detective, suspect, business partner, parents, siblings. A fascinating fictional world can’t exist without them.
They provide fresh points of view, and reveal personality traits about the main character in a fresh way.
Sometimes, a secondary character is an animal. Personally, I love dogs and cats in mysteries; they add a whole new dimension to the book.
Secondary characters shouldn’t all be likeable. Readers need  someone to root against!
They provide sub-plots. If my book is a mystery, then my sub-plots tend to be about romance, or a crisis with one of my secondary characters, which affects my protagonist. I tend to think of my subplots as the underbellies of my books.
Secondary characters can take responsibility of carrying the whole novel off the protagonist.
 Finally, these characters should have something to do, other than popping in sporadically, that moves the story along, enhances the plot, or makes the main character grow and change.