In Edgar Allan Poe's horrific short story "The Tell-Tale Heart," the author uses his language in a precise and careful way in order to create a frightening tale made up of the ramblings of a homicidal maniac.
This is largely a study in human terror experienced on two levels...First, there is the narrator, the maniac, driven by his compulsive hatred of the “evil eye” to kill a man he says he loved. [...] The other level of terror is that experienced by the old man. His terror is made all the more realistic because it is related from the perspective of his tormentor, the mad narrator, who takes sadistic delight in knowing that the old man is quaking in his bed.
The narrator, clearly insane (obvious from the first line of the story) tells his tale of madness—he believes that because of the old man's eye, the narrator must kill him.
Poe, the writer, stressed the importance of methodic structure in creating a story that would bring about the author's desired effect:
Above all else, [Poe] insisted that the writer should make every part of the short story contribute to its total effect. “If his very initial sentence tend not to the outbringing of this effect,” wrote Poe, “then he has failed in his first step.”
So from a writer's standpoint, we understand the importance of the first line: without question, it sets the mood for the rest of the story. However, to keep the mood he has orchestrated elevated to a fevered pitch, everything that follows the first sentence must lend itself to the overall "effect."
In this case, it means that the diction (word choice) is the most important element in carrying out the remainder of the tale in keeping with that first sentence. So Poe employs excellent imagery. He does this through the words he uses, and "stealthily" is one that is particularly effective.
The narrator goes about planning the murder with careful consideration. By the time he is ready to proceed, the reader's sense of horror has been established and only continues to grow. It is the middle of the night and the old man's room is "as black as pitch." He sits up in bed, certain he has heard something and he is petrified.
Presently I heard a slight groan, and I knew it was the groan of mortal terror.
The old man tries to rationalize what could have been responsible for what he heard: perhaps it as a sound coming from the wind in the chimney.
Yes, he had been trying to comfort himself with these suppositions: but he had found all in vain. All in vain; because Death, in approaching him had stalked with his black shadow before him, and enveloped the victim.
The reader can clearly comprehend that the old man's death is imminent. What creates and maintains the suspense in the story is the insane narration that draws the reader along: the narrator's repetition heightens the feelings of fear and dread in the reader. First, he pushes the door open "steadily, steadily;" and, "he had found all in vain. All in vain..." Not only does this repetition promote suspense, but it also creates in the reader the sense of death marching inexorably forward.
The success of Poe's diction is obvious with words and phrases such as "pain," "grief," "death watches," "hinges creaked," "vulture eye," and "fear." The tension in the story grows and Poe's repetition of "stealthily" reinforces the sense of anxiety, as well as the narrator's precision and terrible intent to murder.
I kept quite still and said nothing. For a whole hour I did not move a muscle...
"Stealthily" refers to something secret and sly, deceptive, etc. The repetition of the word here reminds the reader, frighteningly, of the care and time the narrator takes to move forward in his purpose. He remains unmoving for an hour! He is maddeningly patient, lurking in the night. Then the imaginary sound of the old man's beating heart growing ever louder drives the narrator, like an arrow from its bow, suddenly forward to achieve his awful purpose.