Prefacing this by saying that I'm mainly focusing on NPCs with a strong connection to a PC, who are used frequently, and less on those that serve a specific purpose for a single scene or thread. NPCs fall into two categories to me: Background noise and supporting cast. I'm focusing more on the latter.
Even an NPC villain is supporting cast because its intended as a vessel to bring about a character's growth (like a
Sith Lord I used to beat up Sio or the Jedi zealots who brainwashed Elpsis in a big thread I did a while ago). That can be something as simple as the villain of a dominion thread or an NPC enemy your character has a rivalry with (best to make them an actual threat then and give them some wins - a hero is only as good as their foes).
Presently writing a
solo thread for Elpsis' NPC clone
Kyriaki. Somehow, she even got her own Factory subs and NPC posse (and and antagonists). She and Elpsis will eventually meet. It'll be a huge part of Elpsis' story, but Kyriaki will continue being her own person. Hmm I maybe I should make her a PC at some point, as I did with Libertas and Vaena.
Far as I'm concerned, a fleshed out NPC is an PC without an account (and without the benefit of total plot armour). They have their own stories, motivations and personalities.
I use NPCs quite a lot in Elpsis' story. She's a junior officer in Firemane, with her own platoon, so it makes sense that she has people to boss around and take orders from. So over time she got a 'core crew' that is comprised of her main squad. The key members of this group have individual NPC subs (
Nyssa Vykaris,
Diona,
Rhea,
Shikoba,
Natalie Dorne,
Zhaleh) the less important ones are detailed in a
crew sub. Of course, not all of them show up all the time.
They tag along in threads, fight and, you know, interact with her and challenge her viewpoints. The interactions are further fleshed out in blog posts. It's important to me that each has their own voice and opinions and isn't just there to be a drone. One person can't do everything...and Elpsis is new to command. And she has
NPC bosses she answers to...who actually act like her bosses instead of her pals (her CO doesn't even like her at all and Elpsis has to suck it up).
To me it's only self-aggrandising if you're inserting an NPC into a story to fawn over your character and bloviate about how great they are (even if they're generally in favour of your PC, they probably won't do that all the time)...but that's lame because then you're not writing a person. To use your example, it would be self-aggrandising if those miners all just sucked up to Auteme and were just used to exalt her or her faction as a character. I'd just pick one or two, paint a picture in my head of who they are, what their issues are, what they think about the GA/NJO. You don't need to write a novella for background cast. It's not self-aggrandising to write people...interacting with her. Auteme's a diplomat type, talking is what she does.
Her squad has featured in the Ziost invasion and the Annihilation. Tehy fought NPC mooks and otherwise contributed in a way that doesn't interfere in PC vs PC combat.
I proceed similarly with my PCs Kaida, Siobhan and Enyo, though these are less active. With Kaida it's similar to Elpsis, whereas Sio's NPC retinue features a lot of people who'd help her run Firemane and other aspects of her power base.