Once you know enough about your main characters to go on with, you have a whole raft of additional characters who also need to be dealt with. Six realistic, deeply realized, well-rounded, fully developed characters surrounded by five or fifteen or thirty cardboard cutouts do not make for an interesting and balanced story in most cases. Of course, if your plot is one where there are no other characters – the five-person crew of a space transport dealing with an emergency far from everywhere, for instance – then this is not a problem.
Most stories, though, need more than just the central characters. There are usually a string of secondary characters, a bunch of minor characters, and sometimes hosts of walk-ons (those armies that are battling at the gates of Mordor are composed of thousands of characters, even if the readers only actually know ten or twelve of them by name).
The difficulty with secondary characters and walk-ons is that for most writers, the more you know about a character, the more interesting they become. This is particularly true if you are the sort of writer who finds out about characters through their stories, and it can result in the writer paying more and more attention to the increasingly interesting secondary character without realizing it, throwing off the whole balance of the story. If the writer is aware of the shift, and OK with switching stories to a new main character, this is not necessarily a problem…unless it happens over and over again.
What such writers need to realize is that every character has an interesting story of their own. The main character’s valet may be a secondary character in the story you are telling, but he is the main character in his own story, where it’s his employer who is the secondary character or a minor walk-on. It is much too easy to get caught up in the fun of making up a new story to fit the suddenly-interesting valet character, and to drop the half-finished employer’s story, especially if the half-finished story is at a sticky point or some bit that is unpleasant or difficult to write.
Recognizing that every character has his or her own story is only the first part of the solution; the second is disciplining oneself to finish up the current project before starting the new one about the valet. Or the one about the bus driver (who turns up in the next chapter and is also incredibly interesting – now that one is paying attention to her personality and backstory – compared to the supposed main character of the current thing). One has to discipline oneself to take a few notes and set them aside for later, for some other story, or in case they come in handy in the current story, and then go back to concentrating on the main character.
The alternative to gritting one’s teeth and getting on with the original project is to be extremely careful not to develop secondary characters so much that they become more fascinating than the main character one started with. This is, for my money, even trickier, especially since it is both a trial-and-error thing and a process thing – for some writers, starting off with the “wrong” main character is something that happens routinely. It is just how they work, and however frustrating it may be, trying to short-circuit the process will only make things worse. Also, every writer has a different point at which a secondary character is “too developed” and starts to take over and wrap the story one intended to tell around them and their story.
Having secondary characters who have really interesting personalities and backstories can contribute a lot to a novel as long as the writer can hold back from trying to cram in a detailed rendition of every minor character’s personal history and/or from running after every new plot thread that starts as soon as a secondary character takes on their own life. The difficulty in holding back appears to increase rapidly if the author starts making the secondary character a viewpoint for a scene or two, which is a really good reason to limit viewpoint shifts even in a multiple-viewpoint novel. It is much more painful to have to cut out an entire set of scenes from a particular character’s viewpoint than it is to not-write those scenes in the first place, for most authors.
That said, if a writer is having trouble getting into a secondary character’s head, one of the best ways to do so is to write a scene from that character’s viewpoint – for preference, something that the writer knows from the start doesn’t belong in the current story. So rather than writing a scene in which the hero’s valet discovers that someone has tampered with the costume the hero is supposed to wear to the ball, it might be better to write a scene in which the hero’s valet is providing advice to his nephew, who wishes to take up the same profession (valet to a hero), or one where the valet is assisting his elderly mother in making her annual bottling of elderberry wine.
As with the central characters, most writers find it useful to know a lot more about secondary characters than ever makes it onto the page. How much is enough…that depends on the writer and the story, as usual. I know very few writers who start off their books knowing everything about all their characters – most of us invent or discover new bits of backstory along the way (and sometimes, those are the best bits).
And then there are the people who never get into the story at all.
A lot of writers go charging into their plot once they have their central characters nailed down, without ever thinking too much about who else is there…or who else could be there. One of the useful ways of checking for plot, character, and backstory holes is to stop and think about this a little. Where does the story take place, and what does that mean about the types of people your protagonist can run into every day? She works in an office building downtown…so she has co-workers, a boss, the boss’s boss, maybe some subordinates, maybe some folks she knows in other departments (everybody knows the I.T. guy sooner or later…) There’s the barista at the Starbucks where she stops for coffee on her way in, the janitor who always says hi when she’s working late, and that person she’s never met who plays the saxophone every blasted Thursday on the sidewalk eight stories below…right underneath her window. There are the folks from other companies in the same building that she runs into in the elevator or lunch room or parking ramp or on the bus to work. There are her co-workers kids, who have persuaded their parents to sell soccer chocolates or Girl Scout cookies for them at the office. There’s the guy from the United Way who shows up once a year, the UPS delivery person, and the copy machine repair person.
You don’t have to make up all those people before you ever start, because the majority of them will probably never actually appear in the story…but they are all characters who are around, even if they don’t have parts to play this time. I find it useful to keep at least a sketchy list (elevator folks, bus folks, barista, sax player, boss, coworkers) in case I need to mention somebody. Mostly, just thinking about who could be around is enough to let them slip in where they are needed, and if somebody on the list turns out to be more important than I thought, I will usually get an idea of what they are like when they start becoming more important to the main story.
The point is not to show all these people during the heroine’s normal day. Most of them won’t appear at all; in fact, most of them won’t even be mentioned. Knowing that they are out there, though, often allows the author to slip in natural comments or references that they would not have thought of if they were totally focused on the conversation the protagonist is having with her fiancé, without ever having considered who all the protagonist’s casual acquaintances might be.