I believe that I'm somewhat unusual amongst fantasy/sci-fi writers, in that I cut my writers' teeth as a playwright. Coming at writing novels from a theatre background gives me a different perspective on writing than most people, especially since I was also an actor.
I'm not going to pretend that, as an actor, I was much above "competent". My presence onstage would not be a detriment to your show, but that was about about the extent of my skills. So, many years ago, in my acting days, I was in an excellent production of Julius Caesar, playing "Citizen #4".
For those of you unversed in the specifics of Julius Caesar, after Caesar has been murdered and Antony turns the public against the conspirators with his "Friends, Romans, Countrymen" speech, the public goes a little nuts. Thus, four citizens are hungry for some blood, and they know one of the conspirators was a senator named Cinna. They find another guy named Cinna, and proceed to beat the snot out of him, because that's good enough. Citizen #4 gets to explain the logic behind that:
As an actor with only a small bit to do, you do try and make the most of it. Why? Because it's who you are in that moment. I was never a method actor, but I always took to acting with the idea that there's more going on than just your lines. I recall this advice from Michael Caine*, talking about what a director told him when he was in a small part. The director noted him and said, "What are you doing right here in this part?" "Nothing, I'm don't have anything to say." "Of course you do," the director said. "You have amazing, brilliant things to say. You're just deciding not to say them."
Doing this kind of acting crystallized something for me when I was writing. I can't, as a playwright, write a part that would be no fun for an actor to play. And as a novelist, whenever I write a character, even the most minor ones, I can't help but think about making it at least a little more interesting than it, strictly speaking, "needs" to be.
In Thorn of Dentonhill, there's a bit where Veranix runs into two mounted constabulary. These two cops (or "sticks", to use the street vernacular of Maradaine) could have been just Cop #1 or Cop #2. But where's the fun in that? These are still two guys who got up that day, put on their uniforms, got on their horses and went to work. These are two guys who work at night, as partners, in a tough neighborhood where most cops are in the crime boss's pocket. But not these two. These two are a couple of guys who have each others' backs and do their best. These two guys would be the heroes of their own story.
Conversely, in Holver Alley Crew, at one point I jump to the POV of a character who hadn't appeared before and doesn't appear again, partly for the fun of seeing one of the main characters from a completely outside perspective. She has her own problems and concerns, which have nothing to do with what intrudes upon her. Her reality gets affected by the main story, but it stays her reality. And, if I may say so myself, it's a fun bit. It's more fun than had I written it from the main character's POV.
*- This was in a lecture he gave on video, it's not like he told me directly.