Let characters take the world for granted

Why this rule exists

Nothing shatters immersion faster than characters explaining to each other things they both already know, the dreaded conversation that begins as you know. People do not narrate their own world to themselves; they move through it assuming it, the way you do not remind a friend what a phone is before mentioning yours. When your characters treat the extraordinary as ordinary, because to them it is, the reader is drawn into the same easy familiarity and comes to feel like an insider rather than a tourist being lectured. The world feels real precisely because no one inside it is impressed enough by it to describe it, and the reader learns it the way one learns any place, by inference and immersion rather than briefing.

In practice

Let your characters live inside their world without narrating it, mentioning its wonders in passing, the way we mention the weather, and reacting to its marvels with the boredom or irritation of the accustomed. Convey information through action, consequence, and offhand reference rather than through characters telling each other what they both know. When you must get a fact to the reader, hide the delivery: put the outsider or the child in the scene who would genuinely need it explained, or let the reader piece it together from how people behave. Trust confusion for a while, since a reader briefly unsure of a term will happily wait for context, and reward that patience.

Example

"As you know, Captain, our ship runs
on aetheric coils, which, since the
Collapse of '09, only the Guild may
legally forge, on pain of exile."
(No one talks like this. Ever.)

The coil stuttered. "Guild work,"
she spat, and kicked it. "Costs a
fortune and still dies in the cold."
The Captain said nothing. They both
knew what forging your own meant.

When it doesn't apply

Sometimes a character genuinely is an outsider, a newcomer, a foreigner, a novice, and explanation is natural in their mouth. A little direct exposition, deftly placed, is often more merciful than an elaborate contortion to avoid it. The rule forbids the false conversation, not every conveyance of information; the sin is characters performing knowledge for the reader's benefit.