The speaker uses several words out of the usual context to emphasize her journey toward Death.
The first word used in an unexpected context is in the fourth line.
Because I could not stop for Death –
He kindly stopped for me –
The Carriage held but just Ourselves –
And Immortality. (Stanza 1)
One would not expect the word “immortality” used in this context. The fact that the carriage held immortality would seem to assume that if you rode in the carriage you would never die, which is an either wonderful or very creepy concept. In this case it is rather an ironic use of the concept, meaning that death is not really death, but immortality. The speaker feels that when she (or he) dies, she will not really die, but live forever.
In the eighth line, we see another unexpected word,
And I had put away
My labor and my leisure too,
For His Civility (Stanza 2)
We really would not expect to see the word “Civility” here. People normally do not think of Death as civil, which means polite. Really, civil? My goodness, who knew. It is nice to know that when we come to the end, Death is kind and civil. The speaker was in a hurry, and could not stop for him, so he was kind enough to stop for her, but at least he was civil toward her! We think of Death as mean and scary, maybe with a pitchfork or a scythe, but no need! There is no Grim Reaper in this poem. This Death is a gentleman all the way.
As the poem goes on, there are images that seem out of place in a poem about death (like schools), that remind us that this is a commonplace day. The last stanza brings us to another unusual word.
Since then – 'tis Centuries – and yet
Feels shorter than the Day
I first surmised the Horses' Heads
Were toward Eternity – (Stanza 6)
Here we have the word “Centuries,” being compared to the day. It is unexpected because it seems to imply that the woman died long ago, and she is describing her death. We would not expect the word because a century means one hundred years, and it is quite a long time. Centuries have not passed since the poem’s publication, and it is out of place. It implies a general timelessness to the poem, especially given the line about “yet
Feels shorter than the Day” she went to Eternity.
On a general note, you will see that there are many words that are capitalized throughout the poem because Dickinson wants to draw attention to them. The words I have chosen are generally among them. The capitalized words are not normally proper nouns, but they are things that stand out along the journey. The emphasis emphasizes her journey toward Death.
Most of us will not be so cheerful on our journey toward death. Although most depictions of death do not personify it as a person, there are some in literature that do, and occasionally Death is not the cheerless, cruel being society has led us all to imagine!