One of the key ingredients of both good fiction and creative nonfiction is realistic dialogue. Readers certainly know that fact. Writers should know it, too, but they often forget and must be reminded. Actually doing it, however, is hard, and that’s the rub, as the saying goes.
One of my sons-in-law made me more aware of this skill when he began noting aloud the many odd (to him anyway) things my daughter, his wife, said and how she said them. Here are a few of the ones he found recurring in her conversation but that he had heard nowhere else in his life.
“We were meant to be here” (usually spoken whenever we found a parking space close to our destination).
“I could eat more for taste” (at a meal when one was full but enjoyed the taste of the food).
“. . . going around Nicely’s new ground” (another way of saying “beating around the bush” or taking too long to say something).
“. . . got the gaps” (when one was yawning).
“tuna fish” (noting the redundancy of the phrase).
Another characteristic of “real world” conversation is the fact that people don’t always use complete sentences. They speak in fragments, often even allowing single words, knowing that the listener can fill in the missing words from the immediate context of the conversation.
Another characteristic is that people use a lot of slang, colloquialisms, idioms, regionalisms, and euphemisms. All of these, used in your dialogue, add authenticity and color. But they must be genuine and authentic to your setting (time period, geographic location, etc.), or readers will perceive them to be forced or artificial. Moreover, they must be used in moderation. As is often the case with such writing tools (e.g., writing in dialect), a little goes a long way.
The goal is to make dialogue in your writing sound natural and authentic. Practicing verbal voyeurism and applying what you learn to your writing will go a long way toward ensuring that your dialogue is both.