BACK WHEN MY SCHEDULE would allow it, I taught a regular creative writing class at a university in Michigan. Because it wasn't a seminar or a standalone workshop, everyone got to know one another, and the atmosphere was very informal.
We kept the class sizes small, small enough that everyone could fit around a conference table, and, once a semester, I would try the same trick with each class.
I'd put my hat on the table with a small audio recorder inside of it, and I would get the class talking about something I figured they would all have an opinion on: an awards show that had been televised the evening before, or a story that was trending on the news. Then, after the conversation had been going for about five minutes, I would reveal the recorder, play back what they had just said, and ask the class to transcribe it.
Invariably, it was gibberish. Sentence fragments, snorts, indefinite pronouns galore, and, when a sentence actually had a subject, a verb and an object, it would find the most roundabout method possible of getting there.
This amazed the class members because, when they'd been in the midst of the conversation, it had all made sense. But on paper it did not.
In film, it's possible to recreate the chaos of everyday conversation without losing the viewer, because film allows you to throw in the body language, inflection and other nonverbal cues that help keep real-world conversations going. In fact, filmmakers Joel and Ethan Coen are famous for writing just that kind of dialogue.
But in a novel, if you take the time to adequately describe the body language and inflection, you're usually slowing things down so much that you risk losing the reader. And if you employ totally realistic dialogue without adding in those clues, it makes no sense.
Realistic dialogue in a novel, then, is generally something that does not exist in the natural world. People speak in more complete sentences than you or I would use in, for instance, describing how a friend got stranded at the airport. Yet if those sentences come off as too polished, the book begins to sound like writing, and that, too, is bad.
Good dialogue for me, then, always looks a little half-baked. The characters are finishing one another's thoughts. When the end of a sentence is obvious, the character may not finish it, but simply leave it hanging, implied. And sometimes the word choice is not perfect.
You can use tools such as these to suggest everyday conversation, just as a painter can use tools like perspective and shading to suggest a three-dimensional world on a two-dimensional canvas. And like a painter, if you do it well enough, the viewer (in your case, the reader) will happily buy the illusion.
Life is chaos. Your job as a novelist is to find a story in it. And part of that job is imposing order so subtly that the reader accepts it as real.