In "The Tell-Tale Heart," Poe uses a first-person narrator, as he often does in his Gothic tales, to highlight the psychological processes and the breakdown of the protagonist/narrator. The narrator begins by addressing the reader directly:
True! — Nervous — Very very dreadfully nervous I had been and am; but why will you say that I am mad? The disease has sharpened my senses — not destroyed — not dulled them.
He admits that he has a "disease" and is "dreadfully nervous," but he feels that this gives him an advantage. He goes on to explain how he "heard all things" and asks the reader to "observe how healthily — how calmly I can tell you the whole story." This statement, ironically, highlights his insanity precisely because he seems so oddly insistent on his sanity. He recognizes that the reader might find him insane and thus unreliable. He is immediately defensive about how "calm" he is.
The story goes on as the narrator tells his account of a murder. He plans his crime very diligently, even admitting to being "kinder to the old man . . . during the whole week before I killed him." The narrative style continues to read as frantic and excitable as the narrator tries to convince the reader he is no "madman." However, the syntax and tone indicate just the opposite.
After the murder, the narrator hides the body of the old man and cleans the crime scene. He is very proud of his work and feels invincible ("for what had I to fear?"). He even seems joyful about the prospect of the police coming to investigate the room, because he thinks he has outsmarted everyone. However, it's here that his extraordinary hearing comes back to haunt him: the narrator thinks he hears the old man's heartbeat beneath the floorboards, where the body is buried. He relies how he speaks to the police "with a heightened voice" as his frenzy builds. The repetition of the word "Louder!" and its accompanying exclamation show the panic inside the narrator's mind as he is tortured by this supposed heartbeat. He is compelled to reveal the murder by lifting the floorboard to unearth the mysterious sound.
The narrative perspective allows Poe to illustrate to the reader how the narrator talks himself into his own delusions and how his extrasensory "gifts" are a double-edged sword that lead to self-incrimination. The first-person narrative exposes the internal monologue of the character so that we can understand his madness and the irony of the final scene.