There’s a great book on story telling called “Writing Fiction for Dummies” Although the book focuses mostly on novel writing, the principles can be applied to screenwriting as well. One of the more valuable bits of information involve defining your main characters’ values, ambitions, and goals.
Values represent principles that the character believes are true and will not violate for any reason. For example, Louise in “Thelma and Louise” believes that Texas is dangerous so she drives around it.
Ambitions represent desires formed from a character’s values. For example, if a character values family, then an ambition for a character might be to keep a family together at all costs, which is the basis for the hero in “The Godfather.”
Goals represent physical manifestations of an ambition. The hero in “The Godfather” values family and has an ambition to keep the family together at all costs. The goal to achieve this ambition is to wipe out all the major rivals in the other mobs.
This combination of values, ambition, and goals create fully dimensional characters who behave in consistent ways. With stereotypical characters, they behave solely to advance the plot. With characters who have strong values, ambitions, and goals, their behavior seems more real. Even though their actions still advance the plot, it feels natural and not contrived, making your story feel stronger.
What makes major characters particularly interesting is when their values clash with the story. For example, if a character values family above all, how will he react when his family is threatened? How will he react when a family member is the major threat to the family? Suddenly by knowing a character’s values, the actions of other characters takes on a whole new meaning.
Novel writing is a different form of storytelling than screenwriting. Still, you can learn from all forms of story telling to become a better story telling in the screenplay format. Learn as much as you can about screenwriting, but don’t forget about novel writing and play writing too since you can learn from those story telling forms as well.