There have already been a couple answers commenting on the interplay between imagination and reason in “The Fall of the House of Usher” within the narrator, who is also the point of view character. Poe’s short story also plays upon the imaginations of the reader.

“The Fall of the House of Usher” seems to be a parable, or an allegory. What the allegory is for, is open to debate. But the way Poe designs this story to take root in the imagination rather than the logical realist world is by inching the story toward a supernatural zone early. Take, for example, the story’s first appearance of Madeline Usher—the enigmatic twin sister of Roderick. The first time Poe introduces Madeline, she immediately comes across as a spectral phenomenon:

While he spoke, the lady Madeline (for so was she called) passed slowly through a remote portion of the apartment, and, without having noticed my presence, disappeared. I regarded her with an utter astonishment not unmingled with dread--and yet I found it impossible to account for such feelings. A sensation of stupor oppressed me, as my eyes followed her retreating steps.

Logically—reasonably—there is no reason to suspect that Madeline is a ghost or even a supernatural force. The fact that the narrator has never heard of her before does not automatically disbar her existence. The way Poe describes her, however, and the effect she has on the narrator, lead the reader to abandon reason immediately and begin to consider her presence as something far more mysterious.

This is only one example of Poe setting up the abandonment of logic early in the story, to lead the story deeper in the well of imagination by its climax.

Isolated for years inside the Usher house, Roderick's imagination overcomes reason as his seclusion creates a confining and unnatural environment. This bizarre environment engenders his imaginative and hallucinatory attribution of strange occurrences to that of preternatural causes. 

After his twin sister Madeline dies, Roderick is left with his only personal friend from his school days, the narrator. With a terrible sense of isolation now, the crack of the house, like the crack between the living and the dead twins, seems to widen. In fact, Roderick's troubled mind seems to create a pathetic fallacy--the attributing of human feelings and actions to inanimate things--with the decaying mansion and family home. Where there was once a "similitude between brother and sister," now there seems a "similitude" between Usher and his environment. For instance, one night he knocks on the door of his friend (the narrator) and appears to have "restrained hysteria in his whole demeanor." This behavior matches that of what occurs outside the mansion; Roderick opens one of the casements [windows] and exposes a tempestuous night that is

...wildly singular in its terror and beauty. A whirlwind had apparently collected its force...and there were frequent and violent alterations in the direction of the wind.

The narrator closes the casement and urges his disturbed friend not to subject himself to the fury of Nature. In his effort to quiet his friend, the narrator reads aloud to Usher about a medieval knight; however, the narrator now believes that he hears "the very cracking and ripping sound" which Sir Launcelot had so particularly described in the book that he reads aloud. Further, as he continues to read, the narrator is disturbed by a real sound that matches the description of the sound of Launcelot's shield which fell from a wall at his feet. Unnerved by this sound, the narrator jumps up; however, Roderick Usher shudders. He tells his friend that he has also heard this sound. But, with his acute sense of hearing, he has identified it as the sound of Madeline making feeble movements in her casket. Usher tells his friend that rather than being the 

"...breaking of the hermit's door, and the death cry of the dragon, and the clangor of the shield!"

It is, instead

"...the rending of her coffin, and the grating of the iron hinges...and her struggles within the coppered archway of the vault!"

As he has listened to the story along with the accompanying noises in the house, Usher has become increasingly distraught. He insists that Madeline is standing right outside the door. When the door is opened and Madeline does, indeed, stand there in bloody robes, she utters a moaning cry and falls upon her brother as he, too, dies from his shock and fright. In terror, also, the narrator flees.

Much of Edgar Allen Poe's "The Fall of the House of Usher" explores the ways in which imagination and apparent supernatural acts overcome reason and order, replacing them with blind fear. One of the most prominent ways Poe explores this idea is through his characterization of the house itself. Indeed, the house becomes a character in its own right and exerts a sinister influence on the progression of the plot. For instance, Roderick has come to believe that the house has developed some kind of sentience. Moreover, Roderick asserts that this sentience has an evil influence on both himself and his sister Madeline, urging them to follow Usher tradition and commit incest. Obviously, such a notion is not possible within the bounds of reason. However, when the story progresses to its wild and terrifying conclusion (Madeline is buried alive, she escapes and gruesomely brings about the death of both herself and Roderick, and then the house seems to tear itself in two), it's hard not to believe that the house of the Usher family has exerted an evil will upon the characters in the story. In this way, Poe's story gives up on reason and rationality and submits itself to the imagination and the fear this imagination engenders.