I know. I’m the one who had an obstreperous mule (but I repeat myself) as a secondary character, sort of. And one who had a walk on decide he was going to be a main, thus leading to a total rewrite of the MC’s backstory.

You have a main character, the protagonist, the focus of the story you are telling and the person who does stuff, who develops and grows, solves problems, and so on. The lead actor or actress, in other words. This can be a pigeon, a person, a pig, a river, a turtle, a toy boat ( Paddles to the Sea), and other things, but this character needs to the the focus, the one doing stuff and the center of the point of view (but there are exceptions to this – see Sherlock Holmes for an example).

Then you have the supporting actors, ranging from well-fleshed out individuals (the aforementioned mule, Dr. Watson) to the cast of thousands, to David Weber’s secondary characters*. These can be walk ons, sources of important info, minor foils for the MC, henchpeople for the bad guy/gal/whatever. They are secondary, minor, not p-o-v characters. They should not be the ones driving the plot, making the big decisions, dealing with the big problems, and so on. Nor should they upstage the MC unless there is a really good and temporary reason for it, and you make it clear that this is temporary (MC is hurt and turfs command briefly to his executive officer, or something to that effect.)

So, what do you do when you have character creep, and a secondary character starts getting pushy. This might not happen to you, especially if you are someone who outlines, designates roles in advance, and then sticks with those plans. You have probable sorted things out before character development, and your minor characters and bit players say their piece and move on. It tends to be more of a problem for writers who have a rough outline and then launch into the tale, making things happen on the fly. Minor characters seem more likely to try to upstage the MC in those sorts of works. What do you do?

Who is doing what and why? Who is acting, and who is acted upon? Who drives the story, if it is character driven? You need to know the answers to those questions.

First, if a secondary character really seems to be trying to take over and run things, sit back and take a hard look at the protagonist. You might just find that you have the wrong character as the lead. I did this on a short story, and ended up starting over with the former secondary as lead. It worked much better, because he was in a position to act, whereas the former-MC was not. Ideally this won’t happen, because you will have developed the MC to a point where he/she/it/only they know for sure has the chops, position, and basic skills to be the protagonist and get things done.

Pam Uphoff and others (raises paw) have assured a secondary character that he will get his own story later, and done just that. That seems to buy off the pushy person and they are content to return to the supporting role. Note, this is something that might not work for you, and is more for free-form writers. And not all secondary characters will accept a bribe.

You might need to remove the character entirely, or rework the personality, in-world role, or something else about the character to ensure that she stays secondary. Change the MC’s job description so that he has access to the ability to act, and move the secondary to a different task if you keep the secondary. In my current WIP, the short story, the MC is trying to solve a puzzle. Action happens because the MC has a McGuffin, which might not just be a McGuffin. In the WIP book, the MC is out in public, has reasons to talk to people and to go places, and has reasons to act while the main secondary character is off-stage for an extended period.

In stories with very strong, long-term secondary characters, it sometimes feels like a dance, with the central MC, one or two secondary MCs, and then the cast of hundreds. In romances, that’s normal, and you often alternate PoV chapters. I’ve read it done in fantasy as well, often with romance running in the background.

Short version – who should be doing stuff? Is the designated MC the best one for the job? If not, then redo things, reevaluate the characters, and see what went wrong. Bribe the secondary if needed and if that works.

*If you are David Weber, OK. If not, it’s probably not wise to go into long descriptions of really minor figures who are statistically likely not to survive to the end of the book.

Secondary Characters: All the Fun, a Lot Less Work