Beginning to Understand Great Dialogue:
We've all asked teachers or other writers what makes great dialogue and we've all heard the same answer: "Have your characters not say what they really mean." Okay, this is sound advice -- but a little vague. Let's see if we can dig a little deeper.
If we had to use one word to define great dialogue, what word would be "focus." Great dialogue has focus. There's never a word wasted in its intent or execution. Even when a character seems to be rambling, every syllable is there to focus the point, which is that he's insane, or on drugs, or confused, or mistaken.
As a writer, we have to move the story along. Every word of dialogue has to help us do that, but great dialogue does so much more that simply impart information. There are other forms or styles of speech.
3. Imparting information
We realize "imparting information" is in there and most people consider simply giving information as too "on the nose" and very bad. There are times when it can be the perfect form for the moment. If a character has been under interrogation by the police for hours and has finally broken. He might start spouting all the information they need. This is a rare case, however. When starting to write dialogue look at the styles listed above and see which one is best for the scene you're creating.
Whichever style you choose, it has to be chosen for a reason. Listening to a character arguing can be very realistic and entertaining. But when it's over, the reader will be left empty if something hasn't been accomplished story-wise. Make sure the style is used for a reason and is part of building your story. For example, if a man misses a plane and the airline won't bring the plane back to the gate and, later, we learn the plane crashes, which style is best? Perhaps he tries to manipulate them to bring it back -- or even better, he complains loudly. Your character complains about the plane leaving without him, bringing the whole terminal into it. Then, when the plane crashes, suddenly his complaining seems petty. The chosen style has clued the audience into his character and served a purpose in the story.
You can achieve the same "double-duty" with any of the six styles. If a team of thieves is planning a heist, we can set up what's going to happen while showing the differences in each character by the way they approach the planning. Perhaps one guy isn't into planning, likes to wing it. While another wants every detail planned out. We can set up the plan while exposing character. Focusing on the style can help us focus the dialogue and why it's there.
We realize we've only scratched the surface, but defining the dialogue style is the first step to focusing the dialogue and creating great moments in your story.