From the SRP Editor site:
One of the most common mistakes I see in manuscripts has nothing to do with grammar, dialogue, pacing, or point of view. It’s the assumption that some characters matter and others don’t.
You can almost spot them as soon as they walk onto the page. They’re there to hand over a package, serve a meal, explain a clue, or point the hero in the right direction. Once they’ve fulfilled their purpose, they quietly disappear from the story without leaving so much as a fingerprint.
The trouble is, that’s not how people work.
Think about your own life for a moment. Can you remember a teacher who changed the way you saw yourself? A coworker who made an awful job enjoyable? A neighbour whose advice stayed with you for years? Chances are, none of those people were the central figures in your life’s story. They simply crossed your path at the right moment and, without intending to, nudged your life in a different direction.
That’s how supporting characters should feel.
Their job isn’t merely to move the plot from Chapter Seven to Chapter Eight. Their job is to remind readers that the world continues beyond the edges of the page. Every person has hopes, disappointments, routines, regrets, and stories that existed long before the protagonist arrived and will continue long after they leave.
Readers may never learn those stories, but they should sense that they’re there. One of the easiest ways to strengthen a novel is to stop asking, “What does this character do?” and start asking, “Who are they when the protagonist isn’t around?”
The waitress serving breakfast has already had a morning before your detective walked through the door. The taxi driver has somewhere he’d rather be. The receptionist has an opinion about the people who pass her desk every day. The security guard has a family waiting at home. You don’t need to tell readers all of that, but you should write as though it’s true.
Characters become memorable not because they occupy the most pages, but because they feel like real people. Ironically, that’s also how real life works.
Years after we’ve forgotten the details of a particular day, we often remember the person who offered an encouraging word, shared a laugh during a difficult shift, or changed the direction of our lives without ever realizing they’d done so. Stories should leave room for people like that.
After all, there are no minor characters in life. Why should there be any in fiction?
— Mark