One particularly revealing sentence comes when the narrator describes how he went about preparing to murder the old man in order to rid himself of the old man's "vulture" eye. He says,
You should have seen how wisely I proceeded—with what caution—with what foresight—with what dissimulation I went to work!
The first thing you might notice is the repetitive structure of the sentence; the narrator uses the phrase "with what" to begin three consecutive dependent clauses. This word choice seems to parallel and even, perhaps, to illuminate his obsessive, even compulsive behaviors. He is obsessed with the eye, certainly, but his compulsiveness—returning to the old man's room every night at exactly the same time until conditions are just right for the murder (the old man's eye must be open)—is even more startling, I think. He seems anxious to address this aspect of his disease, his compulsions, as being, really, "caution" or "foresight" —something really positive—rather than something associated with a disease or a weakness. His diction, here, also reveals that the narrator is intelligent, especially with his use of the word dissimulation.