A sentence in a story has a large but finite range of purposes it can serve. Here's a roundup of likely candidates:
- Advancing the narrative
- Showing character
- Being beautiful
- Establishing the setting
- Having an emotional effect on the reader, including, but not limited to:
- pathos
- joy
- mirth
- disgust
- arousal
- curiosity
I've probably missed a couple important ones here, but you get the idea.
In good writing, every sentence should do at least one of these things. A passage that does none of them does not belong in the story.
Now, I go a little further out on a limb: In excellent writing, nearly every sentence should do at least two things from the above list.
Part of what I was groping towards in the previous essay is that the way you make a story rewarding is sentence by sentence. In bumper sticker terms, this principle gets framed as "If not now, when?" The place for a story to be a pleasure to read is not 'once I have all the pieces in place.' Your reader owes you nothing, and she's not gonna stick around for that, at least not reliably. You need to offer her pleasures here, now, in this paragraph, and this one, and this one.
This is not quite as hard as it sounds, because, though a mysterious alchemy that philosophers have been struggling to understand since Aristotle (at least), in fiction, nearly all emotions have an element of pleasure. Disgust, anger, sadness, in calibrated doses, are catnip for readers. The one thing no reader enjoys is boredom.
Some days, I might go so far as to compare a story to a fruit tree, with the good sentences--the ones that are memorably funny, or sexy, or beautiful--as the fruit. You can't have the fruit without the tree, but the virtue of the tree is primarily in the fruit it bears.