We open on a forest. In that forest is a person. They are wearing rough, homespun clothing. They have pointy ears.
Therefore magic exists.
At least, in the mind of your viewer. Magic exists, so do dragons, there are probably goblins, people hit each other with swords, they ride around on horses and spend time in inns quaffing ale.
This is fantasy, and all the tropes that come with fantasy. With that one small design choice — a pointy ear — you have described the world to your viewer. They have made lots of assumptions as a result.
But what if, in your world, magic doesn’t exist?
Tough luck, I’m afraid. Because you’ve put in a pointy ear, magic exists. It doesn’t matter that your next line is “Magic doesn’t exist”. The viewer simply won’t believe you. They will start assuming this is a story about how magic is discovered, if you tell them it doesn’t. This, surely, is going to be the big twist of the story!
And actually, if you don’t have any magic in the whole story, they will probably be disappointed. You’ve effectively formed a contract with the viewer. “I have put in a pointy ear,” you have said, “and by doing that, I promise you this is a story about magic.”
So, you don’t want there to be magic? Rethink your design choice. Lose that pointy ear. The trope is just too strong.
But on the other hand, knowing how powerful tropes are makes them incredibly useful as shorthand. You can conjure up whole worlds with a bare handful of words by relying on what most people know.
“The detective sat in his office.”
Without any other work on your part, most of your readers are going to assume that the office is cluttered, it’s probably got slatted window blinds, it’ll have piles of case notes and old newspapers all over the place. There are filing cabinets. The detective probably wears shabby down-at-heel clothing, maybe a trenchcoat. He probably has a drinking problem, he certainly looks tired, he doesn’t have much luck with women. He has a gun, but probably doesn’t use it much. He’s probably American — likely from LA or New York.
That, for most Western audiences, is all summoned up just by those six words. But let’s add one word to it.
“The police detective sat in his office.”
Suddenly that office is different — the room is slightly cleaner, the piles of paper are more organised, the detective’s outfit is tidier. There are probably other neighbouring offices around, and an open plan bull-pen where the lower ranks sit; there’s a coffee machine in that bull-pen.
What I’m trying to demonstrate here is that with a minimal choice of words you can use tropes to do your heavy lifting and set up a default vision of a world and its rules in a reader’s mind. You are sketching a world using the power of tropes. And then you can refine it or deepen it or in some cases contradict it (although some things, like magic in fantasy, are too entrenched for you to ever be able to shift them). You can let the player fill in the blanks. (And therefore, to a certain extent, you’re making them co-creators in your world and making this an interactive experience.)
That doesn’t mean our worlds have to be tropey, and it doesn’t mean that they have to fit particular templates, but it does mean that you can use the power of tropes if you understand how they work. And you’ll know to avoid them if they do things you don’t want them to.
It’s always worth thinking about what conclusions your readers immediately leap to — here are another couple of examples:
“She stood in the airlock.” — Okay, we’re in sci-fi, but not Star Wars space-opera because that doesn’t have airlocks. But not 100% modern-day realistic, because she’s standing in it, so there’s artificial gravity, probably? She might have magnetic boots, but odds are good this is sci-fi, and we’re slightly in the future.
“She floated in the airlock.” — Ah, right, this is much more realistic. It could be a more realistic piece of sci-fi, maybe, like the Martian? It could even be a documentary about the ISS!
Do bear in mind that this is all very heavily based on the cultural background of your audience! If you don’t share reference points with them, you can’t use those references!