Try to limit your cast to a few characters; too many are confusing.
Keep all principal characters together as nearly as possible.
Do not have a character appear in first scene and then not reappear until the last scene.
Introduce all of your principal characters as early in the story as possible, or your audience may form an attachment for the characters shown at start of the story, and when one of your chief characters appears towards the middle of the story it will be impossible to extend the same interest to him as if they have been following his movements from the start.
Use as few minor characters as possible, for the clearness of your story.
The interest should center around just ONE principal character, and never leave that character. If John and Frank love Jane, and your story deals with the hardships and obstacles that John must overcome to win her hand, the interest must all center around John. It is true that Frank and Jane are playing important roles, but it is John's success or failure for which we are wait¬ing. If Jane is a designing woman, and it is her purpose to lead both John and Frank to further her father's success, and THIS is your plot, then Jane is the principal character, and all interest must center around her and her plans and progress. It is hardly possible to write a story without some principal character around whom to center the interest.
Learn to see with other people's eyes, and to feel with other people's hearts.— How TO WRITE A NOVEL, Anonymous.
Knowledge of human nature is the gold which is to be worked into a form of beauty, it is the diamond which is to be cut and polished. Art is that which forms the gold into a thing of use and beauty; it is that which reveals the natural beauty of the diamond to the ordinary observer. A good form, a true art, displays the precious object to the best advantage. . . . Those two are the essential principles of human progress, without whose marriage there can be no children of the imagination.— SHERWIN CODY, The World's Greatest Short Stories.
don't give your character an Irritating personality
Learn to characterize by suggestion, as Kipling does in "The Captive ": "I sat on his left hand, and he talked like — like the Ladies' Home Journal."
Keep each character consistently like itself, unless you are drawing an inconsistent character.
Distinguish between a type and an individual; not an soldiers are soldierly.
Remember that your characters have characters — good, or bad, or mixed, and they will be mostly mixed, if they are human.
Determine on what each character is like precisely, before you write, else the result will be a haphazard vague¬ness.
Don't think that constant harping on a single trait, like the habit of cracking the knuckles, is enough to make a characterization vivid.
Remember that to caricature your characters by exaggerating all their traits is not likely to be convincing.
Do not have too many characters in the foreground; characters in the background will help strengthen the one or two principals.
it requires more ability to make a pleasant character interesting than to bring out a villain, but the villain is much over-worked in fiction.
Differentiate your characters perfectly; it will not do to let them all talk and act by the same set of rules.