The surest way to kill the aliveness of our characters is by insisting that they always make sense. When we follow the labyrinth of most conversations, we discover one constant: people always want something. This doesn't mean that characters are always clear in articulating their desires, or that they're being truthful, or that they even must understand each other. It means they're driven by their motivation.
The purpose of dialogue is to reflect the life and death stakes for our characters. Amidst the most mundane exchange is a yearning for something more. By staying connected to our characters' driving wants, we can write speech for them that reflects an attempt to achieve these desires. Dialogue isn't linear, nor is it logical. With each attempt, our characters are met with antagonistic forces. The tension builds through the scene as each character tries to realize his or her goal.
If our prose feels wooden or transparent, as if we're artificially trying to move the story forward, we can ask ourselves what the characters want. The playwright Harold Pinter wrote elegant human studies that mined the world of the unspoken. At first glance, his plays read as banal conversations, but upon further investigation, beneath the thin veneer of civility in his dialogues live tectonic shifts, life and death struggles.
In a rewrite, if a scene isn't working, it doesn't take long to click on a blank screen (or to pull out a fresh sheet of paper) and write a stream-of-consciousness dialogue. Write it quickly. Surprise yourself by creating what the characters really want to say. It's often in the rewrite that dialogue comes alive. We have a little more security with our structure and we can loosen the reins.
Language is a means of communicating desire. Whether it's to be seen and heard, to gain sympathy, to curry favor, to get information, to feel close, to punish, to win the girl, to hurt, to destroy, to reassure, to secure a position -- we speak in attempt to get something.
But here's the thing: We rarely come out and say what we really want, because within every scene is an antagonistic force. Our characters all have something at stake. There is an underlying tension all the time.
Great dialogue contains tension. It understands what is at stake, and it walks that line. Great dialogue is specific. A single line can tell us a great deal about a character.
I ran into a friend whom I hadn't seen in a while.
"How's life?" I asked.
He sighed. "I want a car with a door that opens on the driver's side."
One last thing: Our characters don't have to speak. If they don't want anything, keep them quiet until they tell you their heart's desire.