We've done the when and the where so it's time to talk about the who of the story. I'll go about this backwards in a way. What I'm talking about is the walk-on characters, the ones you need for some purpose. They might be the cab driver, the waiter in a restaurant, a neighbor who doesn't have a speaking role in the story or a very minot ones. Usually they don't have a name. They have an occupation. That's probably for the best.
Why do I say for the best. Years ago, I wrote a book and my heroine was a nurse. She was giving report to the night nurse. I figured since they were colleagues, I would give the walk-on character a name. The name was a very common one but my pubilsher got a threat about suing them. A woman with the same name felt she was being talked about in a book that had romance. Nothing came of the threat but I learned them to give my walk-ons just their occupation and if a bit of dialogue that fit their character.
I love listening ot the way people speak and so in my head, I have cab drivers, waiters and other occupations that I can use to show they're there and what their purpose is in the scene. So while you're writing and you need a character to come into a scene for a momentary time, think about the people you encounter and give them a description and perhaps a bit of dialogue that fits them.
Like the burly cab drivr, thegrim funeral director, the flirty waitress. You'll pull your reader in and they'll say, I saw someone just like that.