Every character should want something, even if it is only a glass of water.
A concrete way to ensure that you are writing a story — not a scene or a character sketch — is to make sure your character wants something. Give your hero a want or a need, then move them towards or away from that thing. Et voilà! You have a story.
There isn’t much room in a short story. You can’t afford to give your main character two or three things she wants (unless it’s two things that are diametrically opposed). She will have other things that matter to her, of course. It’s just that now — at this moment in her life, the one we’re spying on — she has one overriding want or need that she must resolve.
Secondary characters have wants and needs too, but you don’t have much room to talk about them. If your secondary character is the antagonist (or villain) you can spend more time on their ‘wants’ since exploring them is probably part of explaining why your hero isn’t getting what she wants yet. Otherwise, mentioning their dream in one sentence can be a great way to flesh out secondary characters.
Make Your Characters Want Something
Today, write a story in which you give a character a very specific want or need (you don’t have to spell it out at the start). Move them towards their goal, put rocks in their path, grant or deny their wish.
Give every secondary character a specific need too – even if it never makes it into the story, be sure you know what that person’s dearest wish is.
For more inspiration on this subject, check out Nathan Bransford’s post on the subject.