Details build your story world, and by sharing with the reader what details a character notices, they also build the reader's perception of that character. In other words, details help build a character's voice. But as with most things in writing, more is often less.
We can easily overwhelm a reader with details. Too much description slows down action, and it isn't just what characters see that is important in a scene. It's how they interact with what they see. How they feel about it. In other words, picking details that your character can relate to, helps our readers relate to our characters.
Here's an example from Once Was Lost by the incredible Sara Zarr:
The picture of Jody comes back up. She's in braids and braces and underneath her smiling face is Amber Alert information, phone numbers, website addresses. This is real. The rift in the world--the edge of which I've been teetering on for months--splits wide open, and I'm falling. "I know her," I say to the TV, then look around the room like there might be someone else to tell it to, but there's only Ralph, on the coffee table cleaning his paw.
I don't know the process the author uses. I don't know if she pictured Jody before she envisioned her on the TV, or if the description came to her in this paragraph. But the braids and braces bring Jody to life for me. I don't need to know what color her hair is. I don't care about her clothes. I know her from the braids and braces, and those two small details make her alive and vulnerable in a way that a precise description of everything she was wearing never could. By the same token, I'm not sure that anything else would have brought the contrast of the rest of the world remaining unrocked on its ordinary moorings as the fact that the cat is insouciant enough to clean its paw on the coffee table. And of course, the fact that the cat is on the coffee table in the first place tells us quite a bit.
Every word in that example speaks a volume. It speaks to character and to setting, speaks to emotional tone as well as story. Indeed, it advances story and connects it to emotion.
Here's another example from Sapphique by Catherine Fisher:
The showmen left the village early, before Lightston. Attia waited for them outside the ramshackle walls, behind a pillar of brick where gigantic shackles still humn, rusting to red powder. When the Prison lights snapped on with their acrid flicker she saw seven wagons were already rumbling down the rams, the bear cage strapped on one, the rest covered by contraptions of starry cloth. As they approached, she saw the bear's small red eyes squint at her. The seven identical jugglers walked alongside, tossing balls to one another in complex patterns.
Consider the details there and what they tell us. We know instantly we are in a different world. We know a lot about that world just from the fact that the shackles are gigantic and rusting. We know there are seven wagons so we can picture them; we don't picture four or six because the author left it ambiguous. We know for certain how many there are, and so we trust that the author knows. We believe more completely in the story world just because the author gave us a specific number.
Specificity lends credibility.
But notice that the author didn't describe the wagons. She didn't give us every details. She gave us an overview then showed us what was different, what we needed to know to paint the exact picture that she wanted us to see.
The author directed our eyes in specific directions the same way that a director controls how a movie ultimately unfolds for the viewer.
Here's a final example, the first paragraph of the short story Shannon's Law by Cory Doctorow that appears in the Bordertown anthology.
When the Way to Bordertown closed, I was only four years old, and I was more interested in peeling the skin off my Tickle Me Elmo to expose the robot lurking inside his furry pelt than I was in networking or even plumbing the unknowable mysteries of Elfland. But a lot can change in thirteen years.
Wow. Boy do we know a LOT about that mc just from that brief description. Ironically, the one thing we don't know yet is gender, but does it matter just then? Not really, right? We know inside the character, right down to a vulnerable core of curiosity.
Details are the hardest things to get right in a story. They are what takes the most time to write, because for every detail we include, we have to discard all the hundreds of choices we could have made to include the one telling snippet that reveals the most.
Our readers are impatient. Too much detail will turn them off.
And because they are impatient, we as writers must have infinite patience. We must get to know our characters and their worlds. We must climb inside their skins and their heads and rooms and pull out what they care about, what they need us to know about them.
Ironically, the more we know about our stories, the less we need to write. Instead of putting down a paragraph of description, we can make do with one faceted gem of a sentence.
Let's try it out. Can you write a one-sentence character description that advances your story and setting as well as your character?