Although the characters in your short stories are not real, you have to make those characters seem real, if you want readers to care for or despise them.
Some writers confess that they based this character or that one on a real person, but the best writers would not leave such characters as they are in real life. Fictional characters are supposed to be just that, fictional, and it’s up to you, the writer, to breathe enough life into them to make them ‘seem’ like real people.
If you’ve created a character sheet (a series of questions and answers) then you have a great skeleton to start dressing with flesh and bones. (Read more about this tool in Creating a Character Sheet.) In addition to helping you flesh out a character, this sheet will also help ensure that you don’t make silly mistakes, like describing the same eyes as brown on one page, and as green on another.
Like real people, who grow and change gradually, through life experiences, fictional characters should be allowed to do the same.
While it may seem a good idea to give the reader a full sense of a character all at once, resist that urge. Doing this takes away one of the reasons people read, to continue to learn about the people in your story. By exposing characteristics, beliefs, and other elements of a person a bit at a time, you create mystery, and also maintain reader interest.
Remember, though, because of the length limitations of the short story, you must be careful that you don’t run out of space before you’ve written everything you needed to write about a character.
Since there are no perfect people in real life, it’s a good idea not to create perfect people in fictional life. They won’t ring true, and they will be boring.
Give a character a flaw or two; this is a terrific way to make him or her seem real. When you assign a flaw be sure that it fits the character, and the story you are telling. Making someone an alcoholic, when you’re writing about the reality of marriage versus the illusion of it, could contribute to your theme, but that same addiction might be out of place with another theme. (For more about themes read Write Clearer Story Themes.)
As well as being flawed, each real person is also unique. Give a character a unique trait (or gift) that a reader will remember. Maybe your character is a wonderful story teller who uses that gift to save a person from suicide. Or perhaps your character is musical or artistic, a hopeless romantic or eternally optimistic. Pick the right trait to illuminate your story.
If you’ve added enough flesh and bones, your characters, like people in real life, will want the freedom to speak for themselves. You will know this if you try to make them say something they would never say, and you will hear those wrong words when you read the story aloud, and pause or stumble over the problematic sections.
Have a little faith that the characters you’ve created will lead you in the right direction, to your story’s proper conclusion.