Create a compelling protagonist, or hero. The hero is the focal point of the entire screenplay. He directs the plot, and the other characters are centered around him.
Give your hero a motivation or a goal--something that drives her actions and moves the plot forward. In "The Wizard of Oz" (1939), for example, Dorothy is determined to find her way home. In "The Fugitive" (1993), Richard Kimble needs to find the one-armed man who killed his wife. Sometimes the motivation is found within the plot, sometimes within the character's personality.
Provide your hero with a buddy or a sidekick. While this doesn't apply in every circumstance, it can make for a very entertaining script. Usually it works best if you make this character distinctly different than the protagonist. A lot of humor can be found in the contrast in how the hero bonds with the sidekick. The sidekick can also provide comic relief while the hero plays it straight.
Create an antagonist or a villain. This is someone who is diametrically opposed to the protagonist. While the protagonist has a mission, the villain is determined to thwart the mission and make sure the hero fails.
Provide your villain with a motivation--a compelling reason why he is so opposed to your hero.
Create unique and realistic dialog for your characters. The dialog should sound natural and believable for the setting and the characters. Try to hear the characters in your head as you're writing. Oftentimes it helps to read the script aloud or have a friend read it with you.
Make every character in your screenplay count. No matter how minor the role, every character should make a contribution that drives the plot forward.