I love creating characters. I love love love creating characters. More than anything, character creation is by far my absolute favorite part of the writing process--and no character is more fun for me than The Secondary Character. (or Secondaries, what I call them for short)
Protagonists (and usually antagonists) get plenty of attention—but sometimes I feel like Secondaries are left by the wayside, and slotted in whenever convenient. Certainly these types of characters are necessary, of course, and I do it all the time myself—you've got the walk-ons, the cameo guard or pedestrian, to fill in the gaps of your cast. (I actually don't really consider these types of characters Secondaries at all....they're just Extras. Tertiaries, if you will. :) ) But I personally think that having a good handful of deeply developed, three-dimensional Secondaries will not only make a story richer, but will flesh out both the main characters and the story's world. They're a fantastic way to add exposition to your story without throwing in an info-dump.
When it comes to Secondaries, less is more. Back when I first started writing Legend, Day (my main boy protagonist) had two friends who were essentially his sidekicks. One was a young girl orphan he "adopts" in the story, while the other was a boy who was Day's best friend, someone who wandered the streets of dystopian Los Angeles with him. The two could have been fine if they both stayed in the story, but to me, it felt like I had two 2-dimensional Secondaries when I really could have just one 3-dimensional Secondary. So I combined them into one character. Instantly, I had a more complex Secondary who had her own backstory, inner conflicts, and complicated relationship with my main character. Look in your story and pick out your own Secondaries that you think you can combine and/or flesh out. Do you have 50 Secondaries that would be better as 12 well-developed Secondaries?
Make as long of a profile sheet for your important Secondaries as you do for your main character. Each of them should also be a real person, even if not all of their info makes it into the book—they should have backgrounds, families, prejudices, strengths and weaknesses, and motivational arcs for why they do what they do. Secondaries often have personality traits that are a little more exaggerated than the protagonist's (here's where those tropes come in—the Wise Old Man, the Mischievous Thief, etc). But always remember to ask: why is this character a mischievous thief? Why is this character short-tempered, or angsty, or funny? Did s/he have funny parents? Were his/her parents murdered? Was s/he once not like this, and some circumstance changed him/her? Did the thief grow up in a traveling circus? Was the thief raised by thieves, or did s/he become one out of desperation? Does the thief have doubts about the morality of his/her lifestyle? Knowing the answers to these questions not only rounds out your Secondaries, but they also give you an AMAZING chance to incorporate more world building into your story. What happened in the past life of a Secondary may be some cool culture/tradition of your world that you can share with the reader in an active scene, rather than in a block of boring exposition.
Also, ask how these Secondaries connect to your protagonist(s). How can you use your Secondaries to flesh out your main character? Do your protagonist's motivations hinge on a Secondary? How does a Secondary shift your protagonist's character arc and his/her decision-making throughout the story? How does a Secondary's decisions change your protagonist's?
Does your Secondary bring out the best/worst in your protagonist? What would happen if your Secondary betrayed your protagonist? Why would your Secondary do that? Do they force traits out of your main character that weren't there before? Is your Secondary not really connected to your protagonist at all? Why not?
Lastly, study your favorite Secondaries from other stories. Why do you like them so much? What about that character makes them stick, and how can you do the same for your story? Who is your Boba Fett, Snape, or Finnick? Here are some recommendations from me of stories with amazing Secondaries that should not only be read, but analyzed:
Avatar: The Last Airbender: This may be one of the greatest cartoon series of all time. Why? The characters, particularly the Secondaries. If you haven't watched it, you need to—it is a lesson in fine storytelling (and fine worldbuilding, but that's a whole other topic). Perhaps the best Secondary is a character named Zuko, a young, disgraced prince out to capture our hero (Aang, the Avatar) in order to reclaim his honor and win back his father's love. See? He already sounds as complicated as a protagonist! His personality, backstory, motivations, and inner conflicts are masterfully done. The rest of Avatar's cast is similarly complex, and almost each one experiences some sort of character arc throughout the show. (Don't you dare watch the movie version of this show, by the way. That thing shouldn't exist.)
Harry Potter: Well, duh. Harry Potter has one of the best sets of Secondaries I've ever seen—you could write an entire spin-off series about so many of Rowling's Secondaries. I mean, who wouldn't want to read a separate book all about Snape, or a book about Dumbledore in his youth? When your Secondaries could be main characters in their own right, I think you have a winner.
Firefly: So. Many. Great. Secondaries. In fact, most of the show is dominated by Secondaries, and almost all of them are great. We have Kaylee, the sweet and brainy engine mechanic who also loves pretty dresses; Shepherd, the priest with a mysterious past; Zoe, the tough-as-nails soldier who fought alongside protagonist Captain Mal and whose closeness to him causes tensions with her husband; Inara, the high-class courtesan with her own backstory (that we unfortunately never get to explore in depth...why did this show end so suddenly? ;__; ); Saffron, the brilliant, clever, and complex con artist....the list goes on.
So there you have it! Make with the Secondaries. Good ones are immortalized and become their own forces of nature. And with that, I leave you with: Who are your favorite Secondaries?