Unless your heroes are stuck in a void without a communication device, your story will contain secondary characters. These people make up the backdrop of your story. They will impact how engaging, memorable, and credible your story is. Making the most of them is an important skill. Here's how.
There are two types of secondary characters: supporting characters and minor characters. We'll consider each type separately.
These characters appear multiple times in the story and have some strong impact on the plot or on the heroes. They are usually the friends, mentors, or colleagues of your heroes. They can also be their rivals or enemies.
Supporting characters are there to shine new light on your main ones, to nudge them in the right (or wrong) direction, to help them evolve, and to impact their decisions. Without them, the heroes cannot complete their journey.
That's a big responsibility, and you should prepare them--and yourself--for shouldering it. Make your supporting cast as well-developed as your main characters, even if you don't ever get to include their whole background in the story. Keep a file on each character's background and current situation. Make them round.
The key thing to remember about supporting characters is that they are the heroes of their own story.
They, too, have their desires and needs, their motives, their goals and setbacks, their loved ones and their enemies. They are in your story to serve themselves, too, not just your heroes or the plot.
These people make a short appearance in your story, moving it forward without really making an impact. You still can, and should, make them interesting and credible.
If you feel like a minor character is boring, it probably needs either an upgrade or the editor's scissors. There's a balance to strike, though. If by making this character more interesting you stray away from the story or draw attention away from your heroes, consider cutting the character out.
Secondary Characters Should be...
Interesting. Whenever you write a character, you have the opportunity to make interesting choices about them. What do they want? What do they believe in? How have unique events in their past shaped the way they act today?
Authentic. You will likely need each character to do something to move the plot forward. Make up a good, credible reason for them to do so. Otherwise, the action would feel flat and forced.
Diverse. Beware of creating characters that default to your own worldview or behavior. Different characters have different values, beliefs, political views, and so on. Make the tapestry of your world a rich, colorful weave.
Fleshed out. Don't forget, they're the heroes of their own stories. Plan them in a way that shows this. Do your homework before you write them into your story, even if only a small portion of that background work may find its way onto the page.
Vital. If you question a character's contribution to the story, or if the story works just as well without that character, cut them out. If you hate cutting them out because you're in love with the character, find a way to give them a more imperative role in the story.
Secondary. Don't make them more interesting than your heroes! They can dazzle and attract, but in the end, it's the hero's journey we're following. Don't make the reader regret that.
Secondary characters are a great way to introduce variety and originality into your tale. Have fun populating your story's cast!