You can't say anything only once -- not when you're writing, not if you want the reader to remember. People won't read the words exactly as you intend them, or they will breeze over them; and often your words will admit of more interpretations than you realize, which you rule out by clarifying, angling in, repeating, filling out with examples, adding qualifiers, showing how what you say is different from some other thing it might be mistaken for.
I have long known this about academic writing. Some undergraduates struggle to fill their 1500-word papers because they think that every idea gets one sentence. How do you have eighty ideas?! It becomes much easier to fill the pages -- indeed the challenge shifts from filling the pages to staying concise -- once you recognize that every idea in an academic paper deserves a full academic-sized paragraph. Throw in an intro and conclusion and you've got, what, five ideas in a 1500-word paper? Background, a main point, one elaboration or application, one objection, a response -- done.
It took a while for me to learn that this is also true in writing fiction. You can't just say something once. My first stories were too dense. (They are now either trunked or substantially expanded.) I guess I implicitly figured that you say something, maybe in a clever oblique way, the reader gets it, and you're done with that thing. Who wants boring repetition and didacticism in fiction?
Without being didactically tiresome, there are lots of ways to slow things down so that the reader can relish your idea, your plot turn, your character's emotion or reaction, rather than having the thing over and done in a sentence. You can break it into phases; you can explicitly set it up, then deliver; you can repeat in different words (especially if the phrasings are lovely); you can show different aspects of the scene, relevant sensory detail, inner monologue, other characters' reactions, a symbolic event in the environment.
But one of my favorite techniques is hint, confirm, remind. You can do this in a compact way (as in the example I'm about to give), but writers more commonly spread HCRs throughout the story. Some early detail hints or foreshadows -- gives the reader a basis for guessing. Then later, when you hit it directly, the earlier hint is remembered (or if not, no biggie, not all readers are super careful), and the alert reader will enjoy seeing how the pieces come together. Still later, you remind the reader -- more quickly, like a final little hammer tap (and also so that the least alert readers finally get it).
Neil Gaiman is a master of the art. As I was preparing some thoughts for a fiction-writing workshop for philosophers I'm co-leading next month, I noticed this passage about "imposter syndrome", recently going around. Here's Gaiman:
Some years ago, I was lucky enough invited to a gathering of great and good people: artists and scientists, writers and discoverers of things. And I felt that at any moment they would realise that I didn’t qualify to be there, among these people who had really done things.
On my second or third night there, I was standing at the back of the hall, while a musical entertainment happened, and I started talking to a very nice, polite, elderly gentleman about several things, including our shared first name. And then he pointed to the hall of people, and said words to the effect of, "I just look at all these people, and I think, what the heck am I doing here? They’ve made amazing things. I just went where I was sent."
And I said, "Yes. But you were the first man on the moon. I think that counts for something."
And I felt a bit better. Because if Neil Armstrong felt like an imposter, maybe everyone did.
Hint: an elderly gentleman, same first name as Gaiman, famous enough to be backstage among well known artists and scientists. Went where he was sent.
Confirm: "You were the first man on the moon".
Remind: "... if Neil Armstrong..."
The hints set up the puzzle. It's unfolding fast before you, if you're reading at a normal pace. You could slow way down and treat it as a riddle, but few of us would do that.
The confirm gives you the answer. Now it all fits together. Bonus points to Gaiman for making it natural dialogue rather than flat-footed exposition.
The remind here is too soon after the confirm to really be a reminder, as it would be if it appeared a couple of pages later in a longer piece of writing. But the basic structure is the same: The remind hammer-taps the thing that should already be obvious, to make sure the reader really has it -- but quickly, with a light touch.
If you want the reader to remember, you can't just say it only once.