An adjunct to this rule of bad dialogue is "Show, Don't Tell," another of the most frequent mistakes found in newbie screenplays. You can say more about a relationship in trouble by seeing a husband eye a pretty young thing as he and his wife are walking down the street than by three pages of dialogue about how their marriage counseling sessions are going. Movies are stories told in pictures. So why would you resort to telling us when you can show us? It's so much more economical! You want to make sure the audience knows about a guy's N.Y. Giants past? Show team pictures on the wall of his apartment, give him a limp (from the accident that ended his career, but only if it's germane), sneak it in with subtle references. Want to make sure we know a fight has occurred between two people? Have them talk about anything but the fight. If handled right, the audience will get it. They're a lot more perceptive than you think.