This is some thing I wanted to post on somewhere, now I'm recommending this for beginners, so nobody else pay attention to this entry.
One-Dimensional Character- A character who constitutes the "walk on" parts of the story. These are your waiters, your cab drivers, bartenders, a mother walking down the street, or the random pikeman your hero stabs through the face (though not always). They are briefly seen and do not speak. One-dimensional characters that spend more than several pages in your story should be made two-dimensional. Beside possible subplots, adding realism to an environment, or even promoting general interest, any interaction with a main character serves to flesh out the personality of both characters and add dimension to a scene or personality. Any walk-on that could successfully be made into two-dimensional character should...unless doing so awkwardly upstages an important character/moment or harmfully dilutes the pacing of a scene.