Good writers are always striving to develop dynamic, memorable main characters in their fiction. A story’s protagonists and antagonists should have clear, compelling motives; they should undergo a clear change as the story progresses; and they should be fleshed out enough to be believable. If your story’s cast is full of one-dimensional characters whose lives remain unchanged throughout the narrative, it will be difficult to keep the reader’s attention.
However, you still shouldn’t assume that all of your characters need to be popping off the page as much as the stars of your story. Many settings require you to include a supporting cast of characters who are necessary to the world of the narrative, but unnecessary to the narrative itself. If you take the time needed to develop every minor character who appears, then your narrative can easily turn into a bloated mess that confuses your readers more than it entertains them.
Many writers don’t like to think of themselves as using flat characters intentionally, but there is no need for a flat character to be a boring character. You can easily bring flat characters to life in the reader’s mind by using one or two unique, memorable traits to describe them. Although you don’t want to denote characters with quirky traits to the point where they all become gimmicky caricatures, you can draw the reader’s interest by describing a few interesting, memorable details. It’s easy to picture the waitress in worn out Chuck Taylors, for example, and we can make some assumptions about what she might be like, but this brief characterization doesn’t draw us away from our heroine’s nervousness as she waits for her date to arrive at the restaurant.
Effective writers make their minor characters flat for the same reason that they make their major characters round and dynamic: to draw the reader’s attention and hold it for the duration of the story. Although using vivid and interesting details can help bring minor characters to life, there is no need for an excess of information about them.