Write a critical assessment of Emily Dickinson's "I heard a fly buzz -- when I died --"

At A-Level, it is common practice to address an essay on a poem one of two ways:

1. Chronologically 

2. Thematically 

Either of these approaches are totally acceptable but perhaps in this instance, given that the Dickinson poem is fairly short, it would be better to work through the poem stanza by stanza (chronologically.) 

Always remember that when writing about literature we move in stages: 1) identification of trait or trope (for instance, here, Dickinson's use of punctuation), 2) analysis of what that feature does on a micro-level in the line (in the first line, her use of the dash breaks up the line, causing the reader to pause, and then the second half of the line comes as an even greater shock and highlights the bathos of the first half), 3) assessment of how this idea fits with the whole poem (the whole poem is about the moment death proving less significant than one would expect, so Dickinson's opening line with all its punctuation is entirely appropriate to the whole poem.) 

Tethering individual literary features like meter or syntax or punctuation to a thematic idea and then exploring how that theme works in the whole poem is the building block for analysing any literature -- this should help to form a cogent and nuanced analysis of the poem as a whole.