The strong imagery contained in simile and metaphor is the blood and guts of descriptive fiction. Without it you are working with a mere skeleton, telling rather than showing. Used well, simile and metaphor bring prose to life; paradoxically, however, its overuse can smother the prose and bury the story.
A simile is a figure of speech, usually introduced by like or as, that compares one thing to another:
Emmett is as relentless as a wolverine. Jenny's eyes shine like chips of onyx.
Because a simile's sole function is comparison, it is not quite as evocative as a metaphor. A metaphor does not so much compare as transform one thing to another:
Luanne was a dainty little bird of a woman, given to quick movements.
Behind the house Feldman laid out four squadrons of flowers that sprouted, mute and soldierly, exactly where he had planted them.
Metaphor is subtler and more revealing than simile, evoking imagery beyond the original comparison. Luanne is transformed into a bird, with all the attendant (and unmentioned) fluff and chatter and skittishness that we associate with birds. Feldman's squadrons of flowers suggest something about Feldman himself, evoking military associations and the sense that Feldman always gets exactly what he demands.
With a simile, the comparison stops at the end of the sentence; with a metaphor, the reader's imagination goes on to include all the images and associations that the metaphor implies.
Sometimes you can convert a prosaic simile into a vivid metaphor:
Simile: Emmett is as relentless as a wolverine.
Metaphor: "Emmett?" Judy said. "Emmett is nothing but a wolverine, hateful and relentless. Sometimes at night I think I hear him out there, panting at the edge of the yard."
The metaphor transforms Emmett from a man who reminds somebody of a wolverine into a man who embodies the wolverine's terrifying qualities and who evokes the resulting fear and loathing.
A metaphor can resonate far beyond its original invocation; you can thread a metaphor all the way through a story if you want to. An insistent rain might fall through a story about a failed businesswoman trying to get back on her feet. This kind of recurring imagery is a story's central metaphor. For example, you could fashion a story around an ice-climbing expedition, using it to mirror and vivify the up-and-down emotions that the climber is experiencing in his crumbling marriage. Michelangelo's Pieta could be the central metaphor in a story about a woman artist tending her own dying son.
Writers often discover central metaphors by accident. A friend might exclaim, "I love the kite-flying as a metaphor for Kate's marriage," leaving us to nod wisely while secretly wondering how we ever missed it. Much of our writing comes from the subconscious, and we are all guided by our own personal metaphors, which is why some authors seem to write the same novels over and over. Make yourself aware of your own recurring metaphors, and be careful not to let them become stale.
Whether you discover a central metaphor by accident, or deliberately set out to create one, make sure to weave it subtly into the body of the story, and keep it free of cliche. For example, a five-page story about a young girl's coming of age may be smothered by too many images of springtime, making a simple story seem overblown and melodramatic: blooming flowers, blooming girl. Just because you find some recurring images while rereading a first draft does not mean you are obliged to turn those images into a central metaphor. You may even want to cut some of the images and let the story stand a little more by itself.
Let's return for the moment to our story about Frankie. Notice that a central metaphor is beginning to show itself: light. The harsh fluorescent light of the library; the weak strand of sunlight on his bald head; the observation that full sun has never shone on him. Once you discover a pattern like this, you have a decision to make: punch it up, or tone it down. In this case, the central metaphor of light suits your purpose for Frankie, and you can punch it up a bit by altering Frankie's appearance. Instead of a bald head, give him a stalk of unruly, flame-orange hair that embarrasses him almost as much as his stutter. When the sun shines on him he looks like a lighted match. The recurring image of light in this story is a metaphor for the darkness of Frankie's life, for he has never truly ventured out into the sun. The scene where he is kneeling in his garden with the sun shining on him is a powerful one, inviting the suggestion that Frankie is like the flowers he has planted. Will he blossom like the "ordinary" day lilies, or wither like the delphiniums, which were more promising on paper than in reality?
Let's give Frankie a break and write him a happy ending. He decides he isn't made for raising showy flowers; he doesn't have the right conditions (literally and metaphorically, of course). However, he knows he can grow easy flowers, as proved by the day lilies, so he fills his barren garden with them, discovering how beautiful they are when massed together. As a final act of faith, he gathers the most beautiful of the lilies and heads back to the library to return the book to Andrea. The story ends with Frankie standing in front of the library first thing in the morning, waiting for Andrea to unlock the door. This final moment cries out for a strong image; after all, Frankie has decided to allow the full sun to shine on him at last:
Frankie stood at the library door, flowers in one hand and book in the other, his hair brushed into the red pompadour of a rooster about to announce the dawn.
This final metaphor (the rooster) illuminates Frankie's awkward confidence (his wild red hair has been turned into an asset) and his decision to begin anew. Moreover, "announcing the dawn" brings in a final, reassuring image of light.
THE VIRTUE OF RESTRAINT
Simile and metaphor are irresistible writing devices, but you must take care to control your impulses. We writers are always seeing things in terms of something else (it's called imagination) but imagery can become so burdensome that the readers can't find the story. Metaphors look obvious and simple-minded if rendered too directly and too frequently. Images like thunderclaps during the sex scene, or wolves howling on the evening before the execution are best left behind. And beware of mixed metaphors, in which imagery runs away with itself and ends up confusing the readers:
Arianna shook back her mane of auburn hair. She began to slink toward me, a lioness with the single-mindedness of a rattlesnake.
Are you comparing Arianna with a lion or a snake? Once you've begun with one image (the "mane of hair" already suggests a lion), don't mar it by adding something else. It can be fun, however, to allow your characters themselves to mix metaphors. The character who proclaims "You can't make a gift horse out of a sow's ear" makes for entertaining company.