The question of diction is a subtle one. It refers to style, but also to vocabulary and cadence. That means you could take the question in several different directions. Here are a few examples of how diction affects "A Rose for Emily." Start with the opening lines: " WHEN Miss Emily Grierson died, our whole town went to her funeral… which no one save an old man-servant…had seen in at least ten years."
The term "fallen monument" begins to establish a metaphoric base for the story. Emily's life is one that rests on a social structure that's fallen away; therefore, this early metaphor directly relates to the theme. More simply, the term " man-servant" sets the stage: this is going to be a story about an older or passing social milieu. The same is true of the address "Miss Emily." Simpler still, the phrase "our whole town" relates to the theme because it a) indicates a communal approach, and b) shows how the community is tied in with Miss Emily's life and death.
Moving on, we're told… " It was a big, squarish frame house that had once been white, decorated with cupolas and spires and scrolled balconies in the heavily lightsome style of the seventies…" The specificity of the architectural vocabulary lets us know this is an informed community, one that pays attention to its twists and turns. That's what led people to the investigation into Miss Emily's life and secret.