A charming and unexpected thing happened to me last week. An acquaintance of mine who was reading "The Eminence's Match" told me that his favorite character in the piece was Harat. For those of you who have read the story, Harat is one of the secondary characters - the main character Xinta's exercise partner, who bunks with him in the school dormitory.
When I told my buddy Janice Hardy about this, she told me that she'd had notes from people who loved minor secondary characters in The Shifter - for those of you familiar with the book, one of them loved Kione the guard, and the other one really loved Soek the apprentice Healer.
In effect, you never know when someone might just pick one of your secondary characters as their favorite! Secondary characters can do so much to enhance your story that it's worth giving them some serious attention.
Maybe you've had the same experience I've had, when you read a book and the main character and his/her core group of friends seem to be the only people in the entire world, going about their business. This always disappoints me. I like to see a world that looks lived-in. There are going to be people in the mercantile street, or if there aren't, there should be a reason why. In a city, there are people working to keep the city going. People changing the lightbulbs. Hooligans and police, etc. etc. A world has a population which moves in patterns, patterns which are not necessarily aligned with those of your core group.
Keep in mind as you work that everyone who walks onstage in your story is there for a reason. It may be an institutional reason rather than a personal reason, but even the spear-bearer is there because he's got a job to do. You may not care about that spear-bearer's opinion on these people he's standing next to, but believe me, he's got one. The person who jumps out of the way of your swordsman, the person who gives useful information to your spy, the innkeeper who serves your questing party beer - each one is doing those things for reasons of their own. It's worth thinking a bit and knowing those reasons, because at very least it will help you to figure out their tone of voice and the expression on their face.
I encourage you to think about secondary characters from their own point of view; it's a valuable exercise even if you're never ever planning to give them their own narrative. As you can imagine, I do this a lot - and I actually have a secondary character whose opinions about one of the main protagonists are so important that I've decided to promote her to a point of view character.
Ask yourself what this person does when they're not on stage with the protagonist. What is his/her job? How did it bring him/her into this room? How does he/she feel about being there? Sure, there will be bit parts and people who don't need to stand out in the crowd, but the more interaction this person has with the main characters, the more important it is for you to know what he/she wants and how he/she judges what's going on. Readers can tell, instinctively, whether this secondary character of yours has a life outside the scene they're reading. It only takes a detail or two, or a slight shift in the wording of the dialogue, to give the impression of that life and bring extra dimensions to your story world. Besides, you never know when a secondary character will grow in importance and take on a key role in your story - or give you an opportunity to deepen their character and tighten the story at a later point.
Finally, ask whether and how your point of view character judges the people he/she encounters. I've spent quite a bit of time on the idea of a narrator as a guide; where your narrators put their attention is going to be a great way for readers to tell what's important in the story, and what isn't. If the narrator notices a single individual in the crowd, then that person automatically gains more importance. By having your narrator notice particular types of people and judge them in particular ways, you can create an effect that deepens your world, informs us about your character, and also plays into such issues as theme and plot.
Give your secondary characters some attention. Your readers will thank you for it.