Diction refers to the word usage employed in a poem or piece of prose. When assessing how diction affects a reader's interpretation of a poem such as Emily Dickinson's poem that begins (and is often referred to as) "Because I could not stop for Death," we want to first pay particular attention to the word choices as a whole, then to the particular instances that may seem jarring or insightful given the subject matter, which in this case is a poem about death.
In this poem, Emily Dickinson employs a vocabulary common to much late 19th century verse. The words are simple, yet genteel in tone. The poem follows a meter and rhyme scheme similar to Protestant ballads like "Amazing Grace." This lends a dispassion and sense of calmness to the proceedings as the speaker is encountering her own death. While this rhetorical approach may seem odd to a 21st century reader more familiar with expressive characters, the formal tone of the poem may not have seemed so jarring in poetry of Dickinson's day.
That said, a few particular word choices drive the meaning of the poem in a more peculiar direction. In the first stanza, the speaker notes that "I could not stop for Death," which suggests less of a stoic calmness. This is a character who is likely ambitious and restless. She could not stop.
And yet her personification of Death is described as "kindly" and having "civility," as if Death were a gentleman out to court her in her "gossamer" dress. This is an instance where the diction does shift the flavor and tone of the poem away from a abstract and stoic appraisal of dying, pointing the reader instead to see Death as a romance in which the speaker giddily awaits her immortal carriage ride.
"Because I could not stop for Death—" by Emily Dickinson consists of six stanzas, each consisting of open quatrains, four lines rhymed abab, written in alternating lines of iambic tetrameter and iambic trimeter, a form known as "common meter" and frequently used for hymns and ballads (it can be sung to the tune of "Amazing Grace").
The most striking feature of Dickinson's poetry is her idiosyncratic use of punctuation, substituting dashes where one would ordinarily expect commas or periods. Because this punctuation is so unexpected, readers pause while encountering the dashes, causing them to act as a form of caesura, and creating an odd, start-and-stop reading experience.
The language is deliberately simple and unadorned. One particularly interesting quality is limited use of adjectives and rare and almost jarring using of adverbs (one does not normally consider Death's acts "kindly"). The spareness of the diction and limited use of descriptive terms means that we know few details about the narrator other than her being a middle or upper class woman, and thus focus as readers mainly on the nature of death.
The spare diction and odd punctuation give a distancing effect to the poem, and a curiously abstract feel to a subject normally treated more emotionally.