I have to confess, there's a small part of me that cringes whenever I realize I need another character. Or characters. Especially a character with a name. Because I know, each named character adds a new dimension of complexity, and with that, the risk of losing a reader who doesn't want to keep track of too many characters.
Usually, such a character first comes into the story on a purely functional basis: I need someone for Colin to talk to in this scene. I need Jeric to have a peer group among the Tarian initiates. I need Lesk to be building up his own crew of neighborhood flunkies. Serving the mechanical needs of the story, keeping the gears moving. This is fine, of course, but in and of itself, rarely dynamic.
So I put myself in the frame of mind of my theatre roots: make every role interesting. Think in terms of casting: what would a great character actor do with this part?* How can I make it more than just functional, but without going off on a tangent that has little to do with the rest of the story?
On top of that, it's often in these tertiary characters that the biggest surprises of the writing process is born. Something that starts as functional blossoms into a more crucial role.
One of my favorite examples of this comes from Deep Space Nine**: Damar. Damar first shows up in the 4th season episode "Return to Grace", and his purpose is entirely functional. He's the guy on the bridge of Dukat's ship who speaks, so someone can say those "Target at four hundred thousand, sir" and "Phasers ready" lines. He's the personification of Dukat's whole crew, and little else. But over the course of the series, he stayed at Dukat's right hand as Dukat became the de jure leader of Cardassia under Dominion rule. And once Dukat's madness removed him from that position, there was Damar: now the puppet in charge, and the slow cost that had on his soul became a key storyline in the final season.
Not too shabby for a character that starts out as purely functional.
And that, I think, is the key aspect when bringing any new named character into the mix: investing them with the potential to become something so much more. I can't even begin to tell you how many times I've started a new scene and realized that the best choice of POV character for that scene is someone I hadn't even considered existing when I drafted the outline.
The other key aspect? Keep the names dynamic. I know when I'm reading a book-- especially a sci-fi or fantasy book-- and there's some minor character named Vesslin and another one named Vettlan I am going to mix up which one is which. Heck, when I was twelve and tried to reader Lord of the Rings for the first time, I couldn't keep Sauron and Saruman straight. They were both bad guys with S-r-n name constructions.***
Right now on Way of the Shield I'm working on a scene where a functional character exists to bring a main character from Point A to Point B-- in a literal, "get him on a carriage and deliver him somewhere" way. In writing the scene, I asked myself, "Who is this guy, and how did he get this job?", and in asking that, I found a common thread between him and the main character in the scene, which then helps build his story, and does a bit of worldbuilding work in the process.
Again, not too shabby for a character that starts out as purely functional.
*- An interesting effect of the success of the Harry Potter movies is how it managed to get some of Britain's greatest actors to do little more than oddly-dressed cameos.
**- I do use Star Trek examples a lot, don't I?
***- Tolkien's habit of giving the same character multiple names didn't help me much either.