The narrator in William Faulkner's "A Rose for Emily" seems to be a collective town voice, or a town spokesperson, if you will. Everything the reader learns about Emily is filtered through what members of the town have experienced of Emily themselves. The information is revealed to the reader in pieces, just as neighbors would have experienced it in pieces and then gossiped about it at various times. This is how Faulkner manages to pull off the surprise ending. The townspeople tell the story so the reader knows only what the townspeople know. They didn't know Emily had poisoned Homer and kept his body as a kind of shrine until Emily's death, so neither does the reader learn of it until Emily's death.
To put it another way, the reader gets what the town could experience of Emily from mostly outside of her house. Emily's house is the focal point of the story and the reader learns what the townspeople have experienced of Emily with their senses. The reader gets nothing from Emily's point of view.
When "the ladies" smell "the smell" the reader doesn't infer that the odor is actually Homer's body rotting, because the town people didn't. Who would suspect that? When Emily buys poison from "the druggist" the druggist, apparently, tells others about it and the town voice/narrator tells the reader. The "we," the town, "were not surprised when Homer Barron...was gone," so the reader isn't either.
The reader learns about Emily what the townspeople, from mostly outside of the house, learned by observing and by rare interactions. The reader learns what these "outsiders" know, not what the "insiders"--Emily and her servant--know.
The collective narrator is further explained in the enotes Study Guide on "A Rose for Emily," under the heading Analysis:
By confining himself to the pronoun “we,” the narrator gives the reader the impression that the whole town is bearing witness to the behavior of a heroine, about whom they have ambivalent attitudes, ambiguously expressed. The ambiguity derives in part from the community’s lack of access to facts, stimulating the narrator to draw on his own and the communal imagination to fill out the picture, creating a collage of images. The narration gives the impression of coming out of a communal consciousness, creating the effect of a peculiar omniscience.
The reader, too, then, suffers a "lack of access to facts" as he/she learns about Emily.