What kind of people do you have in your stories. There are a variety of developments for story characters. Let's look at some of them.
Simple character - This character has one distinguishing trait. This doesn't mean he's just a walk on or a minor character. This is a character who has one passion, one goal and one way of obtaining this. When I was writing Obsessions such a character came into the story and became the villain. One trait an obsession with his mother leading to a desire for what he considered revenge. Every act of his in the book was aimed to that end.
Complex character has several characteristics and these can often be in opposition. This is a man or woman who is always at the center of the action.
The flat character has no distinguishing traits. While this is true he must be individualized in some way. Often this character plays a vital minor roll in a story.
The round character may be simple of complex. This is a character who is individual and has distinguishing characteristics. He or she will play a major role in the story.
The Background character is one who has a particular role to play. This character usually has no real traits and may not even have a name. The taxi driver who may or may not speak to another character. The messenger who brings a vital piece of paper.
For all characters except the background one not only must the author decide on their distinguishing trait but this trait must be used to individualize them so the reader will believe they know the story person.