Often we think of good writing as plot, plot, plot, but when it comes to selling scripts, it's character, character, character. Marisa D'Vari at the Writers Store has some thoughts on improving the characters in your screenplay in order to better sell it:

When you, as a screenwriter or novelist, assign a specific personality type to your characters, you can better:

Understand how your character relates to and conflicts with other characters.

Ensure you have a variety of personality types in your story.

Understand each character's phobias and preferences.

Plot a more realistic developmental pathway to your character's goals.

Devise more credible dialogue as your character will speak in accordance to his/her type.

Read all about it here.