I've argued that writing is fundamentally about creating a model. A model, as you may recall, emphasizes some aspects of the thing being modeled while suppressing others. More than simply an interesting theoretical observation, understanding a novel as a model in prose makes the author's responsibility very clear.
Readers, whether by intuition or training, understand that novels are models and are willing to treat everything the author chooses to present as significant. However, it takes effort on the part of the reader to keep track of all the details. Readers expect to be rewarded for their efforts, so the first thing to note is that:
Writer's who use throw-aways squander readers efforts and, by extension, their good will.
You've likely heard of "Chekhov's gun." Wikipedia defines it as "the literary technique whereby an element is introduced early in the story, but its significance does not become clear until later on." You've also likely heard of foreshadowing. I want to suggest something more fundamental that I call The Rule of Two:
Anything to which you call attention in your story must appear at least twice.
For example, if you were convinced by my call to do away with bullies in middle grade novels but you really need a bully, all you need to do is bring the bully back into the story a second time. That second appearance elevates the bully from set dressing to part of the story.
In another case, I wrote a story that involved monsters devouring someone's chickens at a key point. It was a fun scene, but it became much more meaningful after I added the chickens to an earlier scene during a revision.
This is a simple rule, in the spirit of little systems, so don't over-think it. Don't for example, try to work everything you mention during the course of the novel in to the climax and dénouement. The second appearance of something that comes up in the beginning can be later in the beginning or the middle just as well as the end.
And if none of that quite makes sense, think of it as being conceptually green: don't use an idea, character, or setting once and then throw it away in the landfill of squandered reader effort.
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