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.