Anton Chekhov wrote about the idea that, in narratives, only things that are necessary should be mentioned. This principle has become known as Checkhov’s Gun and is used in all different types of media. An example would be, in a movie, book, or story, if a gun is shown hanging on the wall, before narrative ends the gun will be fired. This concept has become so ubiquitous to modern media that it is used as a form of foreshadowing. If an author mentions a detail like a gun or sword on a wall that is an unconscious cue that someone is getting shot or stabbed.
As a Game Master it is important to keep this principle in mind as it is one of your tools to influence expectations. While it may not be a conscious thing that players will realize it is something that is used enough in media that they will unconsciously expect it. Just like most westerners unconsciously expect things to come in threes, to have a beginning, middle, and end, and to have the bad guy dressed in black. This means that unconsciously players expect everything you mention as a GM to be relevant to them and the story.
With this knowledge you can go one of two ways. The first is to accept and embrace this narrative style. This is the easiest method. Let everything that you mention be directly related to the story or the characters. Let the players fill in the rest of the world with their imagination. If you describe a passerby on the road, it is because that person is important and bears further investigation. While this can be very straight-forward you can still surprise your players by not having people be what they seem. Just because they are important doesn’t mean they are friendly or just because they are in a dark alley doesn’t mean they can’t be an ally.
The second way to approach this principle is to treat the world as less of a narrative, and instead treat it as a living breathing thing that the players happen to be inhabiting. This means that not every person that is mentioned has meaning, not every sword that is seen is going to be drawn and not every detail you weave into the description is directly related to the players’ characters. This makes a mystery storyline more dynamic because not every detail is important and the clues can get lost in a rich vibrant world. Throwing Chekhov’s sword up on the wall and then ignoring also prompts players to investigate things themselves, which can lead to a greater sense of agency.
Whether you use the principle of Chekhov’s Gun or not it is something to be aware of, because your players are. They many not know they are expecting it but their experience with this narrative device will shape their expectations. And using your players expectations to bring them to a different conclusion is a great way to add depth to your games.