Good Dialogue.

Dialogue is people talking. Write it that way. Like people talk. Why do people talk? Because they want something. Even if it's only not to be lonely for five minutes, they still want something. If you write dialogue with this in mind, or better still in the back of your mind, you will write good dialogue.

It is, in essence, as simple as that, and this: Dialogue is action.

Get the motives of the people talking, starting with the initiator of the conversation, clear in your head. He wants something; what, and how is he going to try and get it? Then, how is the other person going to react - if he clicks the motive of the other and if he doesn't? And how respond?