Not a writing rule, just something I’ve noticed and struggle with
Secondary characters are real people too. So are villains. If you’re going for real, honest-to-goodness realism and three dimensional writing, consider your secondary, background, and even villainous characters.
One of my past problems is that my writing was populated with excellent protagonists who had hopes and dreams and motivations and background stories, but - especially as I’ve gone back and re-read some of my stuff from high school and college - the story was ONLY about them. Whether it was about one brave person making the epic journey or two people falling in love when they weren’t supposed to or whatever, there were generally the one or two important named characters, and then a host of little background peons.
This makes for kind of boring reading. It’s bloody hard to show off your worldbuilding if your reader only gets to see it through the eyes of one or two people. (It leads to infodumps for no reason, because obviously your character already knows everything about the world. It can also lead to a case of an unreliable narrator, where your character is the exception to one of the big rules or is of a separate/higher class and isn’t affected by some of the laws and problems.) I’m forcing myself, for my upcoming projects, to actually have a small stable of secondary characters that I use often, instead of writing along and realizing “oh…I need someone here for my hero to talk to….” *draws name out of hat*
I also usually made my villains 100% evil all the time. But that’s lame, because very rarely are people evil just for the evulz. Most real villains think they’re the good guys. Even if they ARE evil, and enjoy it…well, they’re the good guys of their own story. (For a really good example of this, check out Soon I Will Be Invincible, by Austin Grossman.) Your villain deserves just as much time and effort as your hero, or the main conflict of your story becomes less believable. It makes your protagonist less of a hero to take down a villain who was obviously written because the story needed one. What does the villain want? Even if it’s world domination, there should be a reason. Write a biography for your villain the same as you do for your heroes.