Sensory details tap into our reptilian brain, the part that responds without filter by any logic. Smells and sounds are often the most evocative of sensory details in writing.
A great example: this opening paragraph of Janet Fitch's acclaimed novel, White Oleander:
"The Santa Anas blew in hot from the desert, shriveling the last of the spring grass into whiskers of pale straw. Only the oleanders thrived, their delicate poisonous blooms, their dagger green leaves. We could not sleep in the hot, dry nights, my mother and I."
Can you sense the danger in those images? She keeps it short and sharp. The words hot, shriveling, poisonous, dagger work with the senses of touch, texture, taste, and sight. Quite a lot packed into a short three lines. And then, we get the people. Who could not sleep. Something really bad is going to happen . . . and it does.
But how do you use sensory details in your own writing? How many is enough, how much is too much?
Two Kinds of Sensory Details
The example from White Oleander uses one kind of sensory detail: external. These descriptions have to do with the environment of the story. The winds, the desert heat, the death of everything but poisonous flowers.
Another kind of sensory detail comes from the internal senses. What a person feels in her throat as she stares across the room at someone she hates. What happens on the skin when we read that letter.
External sensory details are most effective, of the two. You can use more of them without boring the reader or sounding like you're heavy handed.
Internal sensory details are trickier. We readers expect them to be present--they are one way we know what the character is feeling without being told. Show, don't tell, in other words. "He saw that his foot was jiggling, tried to stop it" is more effective than just stating, "He was suddenly nervous."
But they can also become overdone, even cliche.
When to Use Internal Sensory Details
Sensory details are a pause in the momentum of your story. They ask a reader to stop for a few seconds and access an emotion. You don't want to do this a lot. If you do, the story will slowly sink. No forward motion.
But if you don't add any, readers won't know what a character is perceiving, feeling, or thinking, without you saying "He felt sad" or "They sighed, disappointed." (Telling, not showing.)
A thriller writer wrote me about this. She'd been planting sensory details at peak emotional moments in her scene, a good change from telling how her narrator felt. She worked from the simpler ones ("His stomach clenched.") to more complex and imaginative ("A dull pain gnawed his stomach."). She even experimented with metaphorical ones: " A worm of anxiety writhed within her." But was it OK to say a body part did something it couldn't actually do in real life, like the stomach that clenched or the chest that froze? Would it be better to say "her chest felt like it suddenly froze?" I think personally I would avoid using a detail that the body couldn't do, but I've certainly felt a stomach clench, so that works for me as a reader. "Ice ran in his veins" is another one that I see a lot. Not possible, but used!
She tried to avoid the cliches of "Her heart raced" and "The pit of his stomach felt hollow" but she was confused about this. Could she use some of these more ordinary sensory details here and there? Or was it better to use fewer, and make them more creative? Cliches are just descriptors that are overused, so they lose their meaning. Always, more original ones are better (and more work).
She also wondered how frequently to use them--her average is two or three every nine pages in a 350 page manuscript. Was that too many for an intense thriller?
For this great question, I would send her back to her favorite books in her genre. Read a couple of first chapters of thrillers she likes a lot. Underline or highlight whenever she sees an internal sensory detail. (She'll need to read like a writer, not like a reader to see these--they are usually invisible to readers, in skillful writing.) My copy of The Girl on the Train has about two or three per chapter, about the average she's going for. In certain chapters, there are more.
Look at a couple of books. Get an average you can live with. See what these writers use for sensory details--do they go for the complex or the simple.
Problem is, there's no hard-and-fast rule. Most writers develop an intuitive sense about sensory details, based on their genre and audience. Literary writers use more than writers working in commercial fiction.