This is a western from 1957. Some readers will recognize which. I'll give a hint: it's the opening scene. The main character is dead center, moving from left to right. He's the good guy. The others are background characters ... they're here because it matters that this not be an empty saloon; the plot requires lots of witnesses. These characters have no lines, however, because the story is told through visuals — meaning these other unimportant people are extras.
The good guy is the player character.
This is a bad guy. He's not "the" bad guy, he's "the" bad guy's brother ... but he's here to commit a murder that sets up the film's plot. In film parlance, he's a stereotypical fictional person of a specific type, age, social class and motivation. He's a stock character.
In D&D parlance, he's a plot hook. He's there to taunt the party and get them inextricably invested in the adventure, which must be resolved before the party's safe. Note the mocking gaze, the mottled leather vest, the low gun-belt, the flashy wide-brimmed hat, the choice to buy a full bottle of liquor. These are the tell-tale cliches that rush to the front of a DM's imagination — every time. But we can talk about visuals another day.
The stock character is a problem. Savvy players recognize him immediately: because typically he's the only NPC that notices or addresses the player characters. Or, to be more accurate, he's the only character the DM describes in the room at length. "There's a room full of people; one of them is wearing a big brimmed hat. He's standing at the bar, watching you as you enter; his lips curl in a smile." The message is obvious: the DM wants you to address this guy. I can do this set up all day long and every time the players will dutifully do their part. It's like a monkey hitting a lever to get grapes.
But turn on the DM and say, "Describe any other person in the room," and almost certainly the DM will launch into a general description of everyone, and not the one person asked for. "Uh, they're cowboys, having drinks, wearing basic cowboy clothing and, uh, there are a few women at the table with them." This is because the DM doesn't want us to talk to these other people; they're not relevant to the DM's story and therefore of no use to the adventure, except as set dressing. Quite quickly we'll be back with the man and his black hat, conscientiously moving the adventure, um, "forward."
This happens without anyone's noticing it. We're forcefully brainwashed from a young age to recognize how these people appear on film as opposed to those people. There's a deep, intensive science involved in grabbing our focus and directing it, one that we're unconscious of. Look again at the first picture.