Back to figures of speech --
Irony is similar to sarcasm except it's not as biting. It's a gentler way of saying something is the opposite of the literal meaning of the words. Humor is often employed, not loud laughter, but a gentle chuckle.
Metonymy involves putting one word in place of another. It intensifies an idea and expresses it pictorially. It is adaptable and can show a lot in a few words. With lance in hand, the knight headed into the fray. "Tom, just what are you doing? Stop and think." Here instead of saying her brother always plunged into action without thinking you've used a substitution.
Hyperbole - this is an exaggeration that stops the reader and gives a sharp reaction. Mary moved with the precision of a well-oiled clock.
Paradox is a statement that is nonsense or contradictory if taken at face value. This is a difficult figure of speech to pull off. Sally never says anything but she keeps talking.
Onomatopeia - means fitting the sound to its sense. His boots scraped against the concrete. A sound like the scrape of nails on a blackboard sent chills along her spine.
When using figures of speech, choose those that are suitable to the setting or to the characters. Having a city scene and using rural figures of speech doesn't fly. Or having a nurse as a character and using figures of speech appropriate to a ballet dancer doesn't work.