Dialogue is people talking. It’s critical to write this copy in a way that sounds as if real people are doing it. In one of John Grisham’s novels the death row redneck was speaking with a young boy, and their voices were interchangeable. I never “heard” either one. This needed to be fixed. I don’t mean dialect (that’s another trap). I mean dialogue that’s true to the person.
Let’s say your character is an 18-year-old female high school dropout, street smart, tough as nails, cagey and suspicious in any human intercourse. There comes a point in the script when you want to get across certain information, back story perhaps, to advance the plot. Suddenly the tough kid is talking in long, articulate, emotionally revealing sentences. Where did that come from? It’s jarring to the attentive reader. The character loses a bit of credibility.
This is an easy trap to fall into but a hard one for an author to spot. It’s also an easy one to avoid or to repair. Especially in any dialogue scene that involves the next turn of the plot, speak the lines aloud to yourself, then speak the response. Do the words sound right? Don’t worry about wit or quotability. Give the character his own words, the ones the man or woman you’ve created would use. Until you have developed your ear, say them aloud.
Record the dialogue, if it helps, to hear what your people are saying. Eavesdrop on their conversation. How would the high school dropout explain his stalking the girl? Very often you’ll need one of these exchanges to further the action or a “reveal” of some kind to come. Make sure it’s expressed in a way that makes sense to the listener as well as the reader.