Your first chapter sells your book. Your last chapter and storytelling sell your next book.
A few decades back a read a wonderful adventure story. Gripping, fun, interesting geographical and historical details. Then, in the last chapter, the protagonist sailed across the Mediterranean. Except, no sail boat could make the trip the author described. It was a minor detail, but not one I forgot.
A few years later I read the sequel. Same great story telling and so on. Also, same idiocy in the last chapter. This time she had her heroine flying cross country in an impossible way. Well, that was too much. I’ll never look at another of her books.
The moral of the story is that you have to get the details right in fiction. In any science fiction of fantasy, you’ll invent science and worlds. That’s okay. As long as you’re meticulous to be consistent. I quickly loose interest in fantasies where new magic pops up whenever things go wrong.
Many people have told me that Harry Potter’s magic all makes sense if you read all the books. Meh. After a couple of novels with new things appearing without foreshadowing, apparently appearing only for the convenience of the plot, I gave up. They may work for young teens, but not for me. (That also tells you something about how much you can trust my other opinions.)
At the least, research gives you the chance to avoid looking foolish. Too often authors are lulled into thinking that details depicted in movies bear a resemblance to actuality. – David Morell, Novelist and Screenwriter
One speed-bump often inserted by new writers as they try to add details is to put in ones that do not help. You want the telling detail, one that says much more than the words but also one the narrator would notice at that moment. When a cop walks into a room with a body on the floor, she will not initially notice the ominous purple overstuffed chair across the room. She will quickly check to be sure the room is safe to enter and, if there is a question about, that the person is dead.
Making people believe the unbelievable is no trick; it’s work … Belief and reader absorption come in the details: An overturned tricycle in the gutter of an abandoned neighborhood can stand for everything. – Stephen King