One of the things I emphasize to my workshop students, is that the stories they write have to be grounded in reality. No matter how fantastic, how futuristic, how — well, unreal the final story is, it has to start from reality, so that it feels real in its own terms.
It’s like building a house. You can build any kind you want, from a shack to a ludicrous castle, but if there’s no foundation in the solid earth, the house will fail.
If you set a story in a real place, such as the UNC campus in Chapel Hill, you describe it as it is. But you can make changes, for the sake of the story. In The Crivit Experiment, I put a building where there was none, so that I could blow it up. People who were familiar with the campus really enjoyed recognizing the place. People who are not familiar with the campus can still believe in it, because everything around it is real.
If you have an alien person, you must have some knowledge of biology, physiology, human and animal behavior, so that you don’t make gross mistakes, such as by putting eyes where, in a real animal, they would have been vulnerable to predation, and would never have evolved any kind of civilization. Or by having limbs that wouldn’t actually work according to physical laws. You also have to give your alien motives that suit the species, and aren’t just copied from human beings. You have to make sure that the most fundamental of needs — such as breathing, fear of being eaten, need for a reproductive mate, even eating and sleeping — are as they really would be, if such a being were real. There is no need to show these needs, but if you don’t understand them, your person’s behavior will feel subtly wrong.
Some things, such as faster-than-light star drives, have no foundation in reality at all, they are only the products of wishful thinking, and are, in a way, as fantastic as flying to the moon pulled up by bottles of rising dew. But we all know that, and don’t think about it, but just accept it as something to enable us to tell the stories we want to tell. And then, after all, we can still extrapolate from what we know of physics, so that we can portray what such traveling might be like, if it were real. If you don’t know even the rudiments of real physics, nobody will believe you, and they will reject your story.
If you step too far outside of reality, as I have done on several occasions, let your reader know that you are doing it on purpose, in order to tell your story, and not by accident out of ignorance. If you have snow (in the northern hemisphere) in June, at least say that it is bizarre weather for which nobody has an explanation, and let it go at that. If your heroine goes up into a dark attic, when nobody in their right mind would even pull down the stair, just to find out what’s making the groans and thumps coming from up there, she has to have a powerful motive that over-rides the most basic of fears. Mere curiosity is not enough.
You can, in fact, do anything you want in your story. You are, after all, the Creator of its world. But no matter how far removed from the familiar your world is, if its roots are firmly in the real world, your story will be plausible and believable.