Good dialogue adds freshness and life to the page. It enables you to convey information in a lifelike way, rather than boring readers with long narrative passages.
Dialogue arouses emotion in readers, because it provides a certain intimacy which narrative cannot. Readers are overhearing the characters and the characters don’t know it. Since dialogue is the means we most often choose to express ourselves, it is the means most readily accepted by readers.
However, dialogue is like imitation cheese.
Dialogue takes pains to appear realistic, but it really isn’t. It's the product of your conscious craft as a writer, the hardest working element of fiction. Dialogue is like imitation cheese. It looks like cheese, it smells like cheese, it may even taste like cheese, but it's not real cheese.
1. Conveys information,
2. Advances the plot,
3. Makes difficult developments understandable,
4. Defines and reveals your characters.
Dialogue permits fiction to speak not merely to the mind but to the ear as well.
Good dialogue is the pretence of causing readers to think the characters are speaking as they do in every-day life, when, in fact, they're doing nothing of the sort.
What people speak in normal life is only conversation, not dialogue. Dialogue is imitation cheese. It’s fake conversation.
Creating authentic dialogue does not mean reporting speech as it's actually spoken. Leave that to court stenographers. When writing dialogue in your novel, you have to select from the messy flood of actual conversation only the lines, which say what should be said.
Dialogue is conversation's greatest hits.
You rewrite dialogue into language that only sounds like someone speaking, not reproducing actual speech. You are translating the sound and rhythm of what a character says into words. Because of its rhythms, excellent dialogue is easily spoken. It makes for good reading because most readers read dialogue aloud in their minds.
Witness this sharp and lean dialogue between two bad guys in Elmore Leonard's novel, Killshot:
"I popped him one time.” Armand took his hand from the steering wheel and pointed to his mouth. "Right here. One shot."
"One shot, one kill," Richie said. "What'd the guy do?"
"I mean what'd he do you had to blow him away?"
"I don't know. I didn't ask. It's not any of my business, it was a job."
"You blow a guy away; it's none of your business?"
"Whatever he did isn't, no."
"Were you pissed off at him?"
"I didn't know him. Don't you understand nothing?"
"To me that doesn't make sense," Richie said. "Me, I have to be pissed off at the guy."
That's good dialogue. You can hear the rhythm.
It did a lot of work in defining the character of those two guys. Although both are killers, Armand kills without remorse and Richie has to be "pissed off at the guy.” They revealed themselves in their own words.
Elmore Leonard was extraordinary at leaving words out of his dialogue. "You sitting down?" instead of "Are you sitting down?” He comes close to the way people really talk and still achieves what good dialogue should accomplish.
----- (From NOVEL SECRETS available in paperback and Kindle form.)