The subject matter creates tension in "The Tell-Tale Heart" through the way in which there is an increase in intensity and tension leading up to the murder, and then, just when the narrator feels he has committed the perfect crime and he will get away with it, he begins to hear the sound of the old man's heart, which increases in volume until the narrator can no longer bear it and has to confess to his crime. Note how the sound of the man's heartbeat begins, according to the description of the narrator:
The ringing became more distinct:--it continued and became more distinct: I talked more freely to get rid of the feeling: but it continued and gained definitiveness--until, at length, I found that the noise was not within my ears.
The way in which the word "distinct" is repeated to convey the increasing intensity of the mysterious sound that the narrator hears is used to create massive tension because it suggests the madness of the narrator, and yet at the same time how he is not actually mad, because the sound of his victim's beating heart that he imagines he hears could be viewed as his own conscience, which he thinks he is able to ignore through his actions, showing itself and mastering the narrator after all. Tension is thus shown through the ultimate question of whether the narrator is able to commit an act without his conscience torturing him in any way. The sound of the heart indicates that there is no such thing as a conscience-free individual, and the confession he makes at the end of the story shows that every evil crime has an impact on the conscience of the person committing that crime.
The realism is unnerving BECAUSE it could happen. We, as consumers of cinema, might be scared by horror films temporarily, but are comforted by the clear separation between their plots and the reality of our daily lives. The more far-fetched the plot, the easier it is to separate oneself from the possibility of its reality. The genius of Poe is his ability to create fear from real, entirely possible situations. It is possible for a man to be scared to death; it is possible for a man to be buried alive ("The Cask of Amontillado"). Poe is the difference between a werewolf and a serial killer - that which could be real creates in us the most fear.