Specifically, how much detail. How much detail and description you put into your writing is a decision that will profoundly affect the way you story reads. Too little and the world and characters become flat and generic, too much can bog down the whole thing, sacrificing pace and readability for the sake of a more painstakingly drawn scene.
Description, where the detail tends to live, is pretty much by definition not action or dialogue or character development, and doesn’t move the plot. It can tell us about the world or the characters, but so can the voice, the speech, the actions and attitudes your characters express. A certain amount is helpful to set a scene, particularly in science fiction or fantasy, where you want to show the reader that the world of your story isn’t the world we’re used to. Strange planets or alien species or speculative technology or magic might call out for a bit of explanation. That said, most readers fall in love with the characters and their story more than the setting, and you don’t want them to bog down and lose interest. You need to be honest with yourself and admit that you probably find the gritty details of your world more fascinating than everyone else does.
It’s kind of the way my kid’s act in the school talent show was adorable and precious and needed to be recorded and shown to all my friends but the rest of the night was a slog to get through.
I think the best writers don’t dump vast swathes of descriptive pose onto the reader. They break it up and dribble and sprinkle it into the story.
Another good guideline on how much detail to put in, especially if you aren’t working in a wholly fictional setting, is that it is better to be vague than wrong.
If somebody walks into a store, pulls out a gun and demands the contents of the cash register, readers will run with that. If you write that the robber pulled out a nickle plated Smith and Wesson .44 caliber revolver, you need to get that right. Take a few minutes and make sure Smith and Wesson makes a nickle plated .44, make sure you know how many rounds it holds, how big and heavy it is. The world is full of people who know about these things, and if you have the guy pull the gun out of his watch pocket, fire ten rounds then drop the magazine, people who know you can do none of those things with the weapon you described will be wrenched out of the moment. Don’t say a character used to be a sergeant in the Navy, because there are lots of people who know that the Navy doesn’t have sergeants. Don’t have the medic give somebody 100 milligrams of morphine to take the edge off after he breaks an ankle because that dose is only appropriate if he weighs 2200 pounds. The same thing goes for any profession or tool or historical event.
With the internet out your fingertips, it’s easy to check things with a quick search, so you can get stuff right. But if you don’t want to do your due diligence, be vague. The man pulled a gun. The police officer had been in the Navy. The medic gave him something for the pain. Vague is OK, especially if your point of view character isn’t an expert in the field you’re describing, but being factually wrong will break immersion for readers who spot the error.
Detail in fiction is like spice in cooking. Too little and it can be bland, too much can overwhelm. Writers and reader have different tastes and preferences on how much is enough. Melville and Hemingway both wrote stories about a man’s epic battle with a creature of the ocean, but Moby Dick is groaning with description while the description in The Old man and the Sea is sparse and simple
And an incorrect detail is like grabbing the salt when you wanted sugar.
Properly applied, details can add richness and texture to your story. Just don’t weigh it down too much to move, and for the love of Bradbury, don’t put in the wrong ones.