Dialogue transfers information between characters. The writer uses dialogue as tool to advance the story to the reader without excessive narrative. Correctly used, dialogue sets the tone, emotion, and pace of the story. It shows character intellect and often identifies dominate or subordinate characters in a scene.
Here some things to consider when your characters speak:
Avoid lengthy soliloquies. When one of your characters dominates the conversation, what do the other characters do? Do you remember how this feels when someone does this to you at a party or informal gathering? Did you feel like the person was lecturing and inconsiderate? It is likely that the other characters and your reader will tire and your character will be less effective. Try breaking lengthy discourse into segments. You'll get the same message across, while allowing other characters to participate.
Avoid excessive use of jargon. Jargon, or the use of vocabulary peculiar to a particular trade or profession, or use of gibberish confuses the reader. Such dialogue fails to transmit the intended message unless supporting characters or narrative provides intelligible explanation.
Avoid alliteration. Alliteration is the successive use of two or more words with the same letter. Such dialogue, unless it is part of a character's personality, often becomes comical and detracts from the dramatic elements of your story.
Avoid syrupy sweet, affected phrases. Speech in the 1700s often used flowery descriptions and compliments to show genteel mannerisms. In contemporary literature, such behavior makes the character ostentatious and shallow.