"He was clearly furious, and her lack of comprehension only seemed to make him angrier. He bore down on her like a hawk on a field mouse, until she had to bow her back to look up at him. The summer sun was behind him, casting him in silhouette." - Sandra Brown in Long Time Coming.
"Her name was Emily Anne. She was nine years old, with black ringlets, long lashes, and caramel colored skin. Her ears were pierced with tiny gold loops. Her forehead was pierced by two slugs from a Cobra 9-mm semiautomatic." Kathy Reichs in Deadly Decisions.
"The factory recommended ammunition was depleted uranium coated with a carbon-based witches' brew and charged with a tiny droplet of antimatter." - John Ringo & Michael Z. Williamson in The Hero (Posleen Wars Series #6).
There are clear indicators for the different genres. We spot most of them without realizing their significance. "Bore down on her" & "bow her back" are romance indicators. They set a mood. The same is true for any genre fiction. Mysteries require a murder - which means a body. Many scenes will relate to the murder and the clues. Science obviously plays a big part of science fiction stories.
That's not to say that all you need do is including these key elements. Like with any fiction, characters, plot, and setting all must come together to form a compelling story, all the while keeping the genre elements in mind.
