While you are constructing your plot around the protagonist and the antagonist don’t forget your supporting characters. These characters, drawn with a steady but light hand, add richness to the context.
Need more conflict in the middle? A supporting character can foil the best laid plans of the protagonist. She may unwittingly aid the antagonist. He may take the heroine away so that the protagonist does not know how to retrieve her.
The neighbor may be either intrusive or supportive or neutrally antagonistic. Or the neighbor’s dog may inadvertently chew up the one solid piece of evidence your amateur sleuth thought she had.
Or a commanding officer, politician, or neighborhood bully can make life difficult for the protagonist thwarting each move he tries to take.
The way your protagonist interacts with these characters is a way to build depth and reveal aspects of his personality that would not come to light in straight action or deductive reasoning or whatever way your protagonist gets the job done.
There are many possibilities to give supporting characters an active role in story development.
Also, if you are writing with an aim toward a series, these characters in the protagonist’s life part of the enjoyment of your audience. They are waiting to see how the sergeant is going to screw up this time. How Harriet will extricate herself from another doomed relationship all the while pouring her heart out to the heroine.
These characters need more background than walk-ons but not as much as your main action characters. In other words, a scar over the eyebrow is not enough. Give them a history, a personality, quirks, and related behavior.
The time you spend working on your supporting characters and interweaving them in the story line will enrich your work. Plus they are the stuff that brings the audience back for more.