I only had HBO for about six months in the early 2000s, so I missed much of the birth of premium TV, including the show that's on pretty much every critic's shortlist for GOAT: The Wire. Thanks to HBO Now, I've been catching up on it over the last few months, currently working my way through season 3. I'm here to offer the extremely controversial opinion that yes, this is a great TV show.
One of the things that strikes me most about The Wire is the way the writers are able to manage a massive cast, while at the same time giving nearly every character their own motivations, goals, etc., even if they're only on screen for a moment. Nowhere is this better illustrated than with Squeak, who first appears in the episode "Back Burners."
When we first meet Squeak, she's along for the ride with her boyfriend Bernard as he drives up and down I-95 buying disposable phones. In this episode, Squeak serves the role of audience stand-in, asking why they can't just buy a bunch of phones at one store. Through her prompting, Bernard explains for the audience exactly what he is doing and why he's doing it.
But Squeak is more than a plot device**. The writers give her motivations and desires of her own. Frankly, she's bored. She wants to not be bored. It's not the most complex motivation, but it serves its purpose, elevating her into a fully realized character. The fact that her primary motivation is boredom is also commentary on one of the contributing factors to the state of Baltimore's streets, and dovetails with Carver's storyline where he tries to figure out what to do with all the out-of-work hoppers. Kids get into trouble because they're bored. People do drugs because they're bored. And people go for long-ass drives down 95 with their boyfriend to buy cell phones because they're bored.
So, how to apply this to your own writing? Take one of your pieces and look for an unimportant, incidental character. Maybe it's the convenience store clerk who sells your protag a pack of gum. Think about what he or she wants, in general terms and in the context of this specific encounter. Maybe they have a significant other they want to see after work. Maybe they just quit smoking cigarettes and selling pack after pack is stressing them the hell out. Maybe your protag is bleeding profusely from several orifices and it's freaking them out (and also pissing them off since they'll have to clean that up later).
Once you know who your incidental character is, it's easy to work in a seemingly throwaway line or two, or a behavior, that illustrates what they're thinking and feeling. And then you've elevated them from human scenery to another character populating your story.
* A spear carrier is defined as "an unimportant participant in something."
** While Squeak becomes the fracture that Freamon, McNulty, et. al. eventually exploit to bring down the Barksdale crew, I'm primarily interested in the tricks the writers room brings to the table during her introductory episode. Although The Wire, like Breaking Bad after it, is very, very good at not wasting characters.