You have an idea, and you can’t wait to flesh it out a bit. Maybe you have a simple premise that you are getting ready to make into something amazing, or maybe you have details hammered out already. Whatever step you are at the process, you will have to check for plausibility along the way.
This is true in every scene of your writing, and in every turn of your plot. Your reader needs to be able to believe that the circumstances make sense in the world you have provided them with. This means making a point to present the necessary details to your reader, in addition to remaining consistent.
You have likely addressed the big issues in your work. You understand that you can’t have a human simply jump into the air and fly in an earth-like setting without explaining why this can happen at some point. However, have you thought about the details? For example, if this flying person flies over people’s heads, do they feel a breeze? Is there a child who gets hurt after jumping from a tree branch because he witnessed this event and wanted to give it a try himself?
The little parts of the story that can make or break things are important. Left unaddressed, they can leave the reader wondering what’s happening, and may cause them to put down your work to ponder these situations. They may not view the fictional world you’ve created as a viable one, and this can make them question the rest of your story.
The further you travel from the world the reader lives in, the more rules you will need to create in your fictional world. For example, if you set up a small village on a small desert planet, you will likely need to address things like the temperature, how gravity works (or doesn’t work) in this setting, and where the individuals in your story get their water from, as the reader will wonder about them and be unable to dive into the story without these details. You will then need to make sure you stick to the rules you’ve established to keep your story believable.