These days, fiction writing is a somewhat less formal affair than it has been in the past. All-knowing narrators who set the stage with passages of elegant prose have largely made way for limited omniscient narrators who share the voice and attitudes of a single character. Those characters voices can still be elegant, flowery, and perfectly correct, but there is more room than ever before for informality in prose. It can be as informal as dialog, if an author so chooses.
The bad news is that this doesn’t make it any easier to write fiction. In some ways, it makes it harder. Strategic breaking of rules requires both an ear for the language and a solid understanding of the aforementioned rules. If you just don’t know how to write then believe me, we’ll know.
The one-word sentence, or more generally the incomplete sentence, is a tool you can use for emphasis. Like this. It’s short, to the point, and easily digestible. It calls attention to itself and its brief content, making that content stand out. When done correctly, it is a way to shamelessly exploit a reader’s emotions. Bam! Right there.
Variety is the key to so many things in life, including sentence structure. Longer sentences are slower, but sometimes that’s exactly what you want. A long sentence of description, for example, can convey many details about a particular object while enhancing that object’s importance because you took the time. Now the reader has his attention focused on the big rubber ball you have just described in detail, from its smooth surface to its patriotic coloring. Perhaps there’s a red star in the center, cut in half by a thin raised line that runs around the circumference of the sphere. The line reminds you a bit of the equator, while the blue circles at the top and bottom are a bit like the north and south poles.
Okay, I’m watching. Now what’s the ball going to do? I’m primed for it to do something, and if you use a shorter sentence to describe that action, that action will stand out. Actions should stand out. Moreover, the give and take keeps the reader on his toes.
The ball disappeared. Vanished. Into thin air. It could have vanished into thin air, but I want to emphasize that it vanished. I want you to feel that it vanished. It’s not there anymore. Gone. Like it never was.
There’s no right way or wrong way to use the one-word sentence. Some authors almost never use it. Some overuse it. This is a tool. Somewhere out there is your own personal style. You find that style after you embrace the rules, then spend years of practice learning how to break those rules.