1) Create characters that are not perfectly predictable – give them contradictions and moments when they act in a surprising manner.  This does not mean you should allow them to get out of character.  Push them to the limit of their personalities.  Test them.  Let them evolve so the reader is both surprised by and accepting of their actions.  By both keeping your characters within their personal limits and challenging them, you will create people whom your readers will not want to leave.

2) Avoid the temptation of making your protagonists larger than life.   Perfectly charming, witty, generous, and heroic characters come off sounding more like the writer’s fantasized life for himself than a real character.  Endowing characters with small flaws won’t jeopardize their likability; instead, it will give them a humanity – and a vulnerability – that will attract your readers. 

3) Choose your character names with care.  While you need not pick character names with inherent meanings (for example, a basketball player called John Netster), you should understand that your readers will have more difficulty keeping track of your characters than you do.  Avoid main character names that begin with the same letter – unless you intend this effect.  Vary length and internal sound.  Characters named Patsy, Betsy, and Betty could be confused, as could John, Jack, and Jay.  If every character has an unusual name, you can run into the same problem.  Your aim is clarity.  Anything that will potentially confuse your readers should be changed. 

4) Can multiple characters be condensed into a single character?   If you have a large cast, you should ask yourself if multiple characters serve the same function.  For example, if you have four different police detectives, could you combine them into a single character?  If both a teacher and a friend act as sounding boards for your protagonist, do you need both?  Unnecessary complication will detract from the readability of your manuscript. 

5) You don’t need to include everything you have imagined. Knowing your characters and their motivations does not mean you should include everything in the story itself.  As in real life, let their history be known in the context of events.  Your knowledge of the characters will emerge in subtle ways, giving them depth without overdone exposition.

6)  Memorable characters are always flawed.  It’s up to you, the writer, to find the balance between a likable character and his shortcomings.  The protagonist of your story should not be perfect but human, with weaknesses and idiosyncrasies.  If you doubt your readers can empathize with an imperfect person, look at Quasimodo in The Hunchback of Notre Dame.  The trick is to make these flaws inconsequential in light of a stronger, more positive quality – such as ethical behavior, or heroism, or passion, or longing for friendship.  Note how any of the positive characteristics I’ve listed can also be a component of an antagonist .  For example, an extremely ethical character might adhere too blindly to a set of rules, thereby misjudging or betraying others.

7) Give characters “space” appropriate for their roles.  Don’t spend a lot of sentences or paragraphs developing a character who has no bearing on the outcome of the story, but also be careful not to “cheat” a supporting role.  We don’t need to know the motivation and life story of the cashier who rings up the main character’s purchase (unless it is important later), but we do need to understand the personality of the main character’s best friend. 

8) Develop your antagonist.  Give him a past and motivation as well as personality quirks that make him seem real.  The more your readers understand about your antagonist, the better they will remember him.