Minor characters are the second rung up from walk-ons. They occupy many of the same niches as walk-ons – cab driver, waitperson, store clerk, army private, city guard, maintenance worker – but they’re not just there in the background. They interact with the central characters in more ways than their obvious functions; they chat, they draw out backstory and main-character characterization, they even drop key clues sometimes. They’re almost secondary characters, except that they only show up once or twice in the whole novel.
It is fatally easy to allow minor characters to fall into stereotypes or reverse stereotypes. They have so little dialog and stage time that the writer can only provide one or two character traits, and even those often have to be exaggerated in order for the reader to register them. And the more unusual those one or two character traits are, the more attention gets focused on the minor character, leading the reader to expect the character to return as, at the least, a recurring secondary character. If the writer doesn’t want this to happen, it’s often simplest to leave minor characters as basically walk-ons-with-dialog, and not attempt to give each and every one of them a distinct personality.
On the other hand, it is not unusual for a writer to get fond of or interested in a minor character in the process of creating a suitable personality for them, which can quickly promote the character to secondary or even major character status. This turns out to be a really good way of creating secondary characters for many writers, because characters who grow on the writer this way are ones the writer doesn’t have to work as hard to get to know. The catch is that if it happens too often the writer ends up with a zillion interesting secondary characters crowding the book and demanding their own subplots, or even wanting to be promoted to major character. This can get really hard to juggle, and can even split the reader’s attention with so much visual clutter that they can’t follow the plot. In extreme cases, the writer can lose track of the plot completely while following minor and secondary characters down their intriguing rabbit-trails.
About all one can do is keep an eye on the situation and rein oneself in when it comes to minor characters. Yes, the busy tavern will have twenty or thirty people at tables, six barmaids running back and forth with drinks, two people pouring drinks, and a cook dishing up bar food, but they don’t all need personalities and dialog. Your characters are probably going to sit down at a table, order, talk among themselves and drink, and leave; that means that everyone but the barmaid who directly serves them is a walk-on that can be ignored or covered with “the tavern was crowded” (or, if you must have more detail, “eight full tables were keeping six barmaids hopping; the actual characters were lucky to find seats in the corner”). The barmaid who serves them is probably a minor character who can get by with a surly “So, what’re you all drinking?” or a cheerful “ Back again? What’ll it be tonight?” and a “Here you go” as she hands out their order.
One way of keeping walk-ons and minor characters in line is to concentrate on your viewpoint character. Your POV is an individual who, like you, will not and cannot notice everything and everyone in a given spot, and the things and people your POV character notices are going to say more about your POV than about the minor characters he/she is noticing. An artist POV may mention the barmaid’s interesting face or the clashing colors one of the patrons is wearing, but the significance of the mention isn’t about the barmaid or the patron – it’s about the fact that many artists see and notice things like faces and colors. Your botanist POV might be in ecstasies over the rare astragalus species the barkeeper has growing in a pot on top of the beer kegs, and not notice any of the people at all, and the really tense and thirsty POV may only notice whether the barmaid is quick or slow about getting their drinks.
If you need to know exactly who is in the bar, why they’re there, and what they’re doing, make a list. “Patrons: Bald blacksmith with mustache & blacksmith’s journeyman, in for a quick drink after work; sleazy guy with long brown hair, looking for a mark to con; …” Your readers don’t actually need to know most of that.
The exception, of course, is if your characters are going to run into one of the bar patrons later, because the sleazy guy tries to con one of them or they need something mended by the blacksmith next day. In that case though, a) the blacksmith or the sleazy guy are at least secondary characters, not minor ones, and b) readers still don’t need more than a mention of their presence unless what’s going on in the bar is relevant to what happens later. If the bar scene is the supposed to be setting up for something in a later scene, or if it’s supposed to be the blacksmith’s or sleazy guy’s introduction to the characters, then more relevant stuff probably happens and those secondary characters will play a part in the scene. If all that happens is that your main character or POV sees the blacksmith have a drink and then leave, you may not even need to mention it unless the blacksmith is going to need an alibi for a murder later in the story.