Dialog allows characters to interact with their setting, with each other, and with the reader. Dialog challenges the writer in both use and execution, both technically and artistically. The rules of use follow the same pattern as the use of description and exposition, but because dialog feels like a different animal the temptation to treat it differently can be excruciating.
Dialog serves the same purpose as exposition, and can be treated the same way. There is nothing intrinsically special about dialog to set it apart from narrative or descriptive exposition, only the “voice” and perceptions of the speaker should differentiate it from the surrounding text. As small a thing as this is, it serves the very important purpose of introducing variety into the reader’s experience and serves to break up descriptive and narrative passages.
Dialog may partially substitute for description, and descriptive dialog should be treated the same way. Having characters witness and describe a setting or event allows the writer to introduce uncertainty in the form of selective perception and expression. Descriptive passages by the “narrator” carry the weight of law, but characters have the same flaws as other people. This ability lets the writer focus the reader’s attention on a single portion of the setting or events and cast it in a specific light.
Dialog should never present information that the reader already has or is about to receive. Although dialog serves the same purpose as description and exposition, readers expect characters to do more than simple recite their experiences and describe their surroundings. If the details of a scene have already been given to the reader, characters can assume knowledge and refer to things without repeating the details. If a reader is about to receive this kind of information, characters need not go into it at all. Never make the reader read twice what he only needs to read once. A particular danger here is the “recap report”, where one character repeats the action of a previous scene to another character. The reader was there, don’t make him read it all again.
Dialog should always serve to move the plot forward or to advance character development. Real people spend hours a day in repetitive, boring, mindless, and meaningless conversation. The last thing a reader wants is to be subjected to the same experience in a novel. Characters can and should display “realistic” behavior such as engaging in small talk and reviewing their grocery lists, but unless that small talk reveals a dramatic development in their relationship or that grocery list suddenly turns out to be a treasure map, the writer should gloss over the experience in favor of a few narrative words. The reader’s time and attention are precious; do not waste them on trivialities.
Dialog should always be accompanied by an identifier. Novels do not contain pictures, verbal cues, or other identifiers. Readers must put an effort into identifying the speaker of any given line of dialog, and readers often have a very different mental picture of a scene than does the writer. Simply identifying the speaker for the reader in every passage of dialog promotes readability and helps the reader more accurately reproduce the story. This may be done by linking dialog to descriptive or narrative sentences explicitly featuring a character, or by including the underused “John said” at some point in the speech. Although grammatically permissable, dialog should never be left without an explicit identifier.
Long speeches have the same effect as long descriptive and narrative passages. Dialog has its own Rule of 4 to help break this up: a paragraph may only contain four lines of dialog, and one of those must refer to the setting. A second Rule of 4 follows: for every four continuous lines of dialog, an explicit identifier must be used. Your readers will thank you.