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.
Using a citizen of Jefferson as the narrator makes the diction representative of the South's traditions. The theme of death is related to Emily's father and Emily herself. Her father's death leads Colonel Sartoris to grant Emily's taxes be paid "into perpetuity". Emily is vividly described: "She looked bloated, like a body long submerged in motionless water, and of that pallid hue."
There are many examples of the theme of the decline of the Old South after the Civil War. In the Old South, men honored a code of chivalry while the women were innocent models of morality and ladylike behavior. Even when the smell around Emily's house became obvious, Judge Stevens refused to allow Emily to be questioned, asking "...will you accuse a lady to her face of smelling bad?"
The younger generation has abandoned most of the genteel ways and aristocratic behaviors of the older generation. By the time Emily dies, "garages and cotton gins had encroached and obliterated even the august names of that neighborhood."
The theme of isolation Emily has from the town is also enhanced by Faulkner's word choice. At her funeral, Emily is described as having been "a tradition, a duty, and a care; a sort of hereditary obligation upon the town." This expresses not only Emily's isolation but the need the community seems to feel to concern itself with Emily's business.