This was a question on writing stackexchange, and even though it was two years old, I found it fun to think about and I wanted to try to answer it. Question in more detail was:

The Harry Potter series has been a major hit, and it is nowhere near to being scientifically possible. But I’ve noticed that I can’t seem to just “let things be” in my writing, as J.K. Rowling has. She knew her readers would accept what was presented to them as they were, because it was fiction. But I keep feeling the need to explain everything in hyper detail, and that means I have to research some crazy things.

It takes so much effort, and half the stuff even I don’t understand. How am I supposed to explain the way my world works, without losing myself and my readers? Is there a simpler way to explain complicated things without having to spend hours looking it up?

What I read from his post was “How do I know when I need to explain the rules of the world and when I can let them be?” Here’s my answer (reposted).

A short answer: if the reader wants an explanation, then provide an explanation.

A theoretical answer: it depends on the reader’s expectations.

Reader expectations, firstly, depend on the book the reader decided to read. Looking at the cover of Harry Potter and the Sorcerer’s Stone, you see a boy flying broomstick and a unicorn galloping in the background. That’s the book the reader chose to read. They already expect and accept magic. A mystery novel will have different expectations than a romance which will have different expectations from a nonfiction book and so on.

Reader expectations, secondly, depend from page to page, based on what questions are being raised. In Harry Potter, J.K. Rowling gives us the first hint of magic, on page 3: “It was on the corner of the street that he noticed the first sign of something peculiar – a cat reading a map.” We don’t wonder why or how is a cat reading a map, but rather who is the cat. And that’s the question she forces us to think about over the next 5 pages as she describes this strange cat interacting with Mr. Dursley. On page 8, we learn who it is, and even after the cat transforms into a human, never did J.K. Rowling explain how she did that —- because that wasn’t the question on our mind.

If J.K Rowling wanted us to wonder about how will the cat morph into a human, she could have changed how she introduced the cat: It was on the corner of the street that he noticed the first sign of something peculiar – a cat reading a scroll, titled: a potion to turn into a human. Now, as the reader, we want to explore how will the cat do that, and now we expect explanations on how that will happen.

So, read your writing and ask, does the reader want me to go into hyper-detail? Do they expect it? Are they reading my novel, in part, for the next interesting detail about my world?

And the best part is, whether or not, they are – you can change the question on their minds. If you don’t want to go into these detailed tangents about the physics of your world, then don’t make that promise to them early on. If you do want to, then do that; rise an interesting question for them to wonder about, then give them a satisfying answer.