"I heard a Fly buzz - when I died" opens with an intriguing statement that draws the reader in immediately. The poem recounts a story from an unusual speaker—one that's dead! Instantly, then, the poem feels intensely paradoxical, the speaker offering a faithful account of the moment of death after the fact (even though that is, of course, impossible).
In just one short line, then, the poem establishes its main juxtaposition—between life and death mainly, but also between mundanity and profundity. Dying, especially for this speaker (who can reasonably be understood as living/dying in the 19th century, the same as Dickinson), is meant to be a profound and serious occasion. Yet, as the poem will go on to explain, this nagging, annoying fly seems to be literally and metaphorically getting in the way. This first line places the fly front and center in the poem, so that the speaker's focus on the fly in turn becomes the reader's.
The meter reflects the fly's irritating noise, with two stresses in succession varying the poem's iambic meter almost immediately:
I heard | a Fly | buzz - when | I died -
The close stresses make the line itself noisier, suggesting the fly's buzz. If instead it was perfectly iambic, such as in the following manner,
I heard | a buzz- | ing fly
the line would be far less evocative. As it stands, the poem begins by evoking the disorder the fly creates.
The poem's juxtaposition between life and death, and between mundanity and profundity, is also developed by the first line's caesura. On one side of the characteristic Dickinsonian dash there is the fly, and on the other the speaker's death. On one side, life (in the form of the fly)—on the other, death.