I consider these to be the most important aspects of writing effective dialogue:
1. All your characters will have a common understanding of how the world works as well as a basic knowledge of their society. Say “Seinfeld”, “Levis”, or “Starbucks”, and most Americans will know exactly what you’re talking about. Pop culture references are not necessary for effective dialogue, but going to great lengths to avoid using one inhibits your writing. If you mean Walmart then say Walmart, not “that store”. Avoid generic terms if they’re going to dilute the strength of what your characters say.
2. Your characters either know each other or will learn about each other. They therefore have or will have a shared history. How you demonstrate this in dialogue helps the reader to subconsciously pick up on just how long your characters have had a relationship. This includes all conversations with secondary characters. Here’s an example from my story, CAROLINA NORTH, where the main character is talking on the phone with her sister:
“All shall be revealed in due time, but until then–” There was a rustling of movement on the other end.
“Lila,” Carolina sighed. “The whole zipping your lips thing doesn’t work well over the phone.”
“You got it.”
“Because I know you.”
“So what’s the problem?”
These two have a history (obviously, since they’re sisters) that is reinforced by Carolina’s understanding of what’s a habitual quirk of Lila’s.
Do not have your characters overexplain something to a friend/sibling/etc. that he or she would already know. This can be both obvious (if they’re both teachers, don’t have one explain to another how a lesson plan works) or subtle (if your character is telling a story about her high school years, mentioning the name of the high school specifically, “When I went to Strack High…”, is unnecessary, especially if their longtime friends).
Remember: there’s a reason why your characters are talking to one another. When two people enter into a conversation, they usually have something immediately in common, which is some level of ingrained politeness or social grace as well as a desire not to come across sounding like an idiot. Beyond that, your characters have chosen to maintain certain relationships, thereby increasing their shared history. If you need to let the reader in on why something occurred, don’t have your characters explain it in dialogue if it’s already part of that shared history. Character A knows that Character B knows, and Character B knows that Character A knows.
“I had to leave the party early,” Jenny told her coworker, “since I always work on Sundays.” [Wouldn't her coworker know that?]
“I saw Dad while he was working at the dental practice,” Jenny told her sister. [Couldn't she just say that she visited him at work? We're assuming the sister knows what their father does for a living, too.]
“I made the zucchini with tomatoes, onions, and garlic just the way you like it,” Jenny told her husband. [If that's really the way he liked it, would she necessarily have to point out what all is in it?]
