The way a story is narrated and the point of view by which the story is expressed says a lot about the story. When a story is told in the first person we see the world through the eyes of one character, usually the main character. When a story is told in the third person we see the world through the eyes of another person not the main character and the opinions of this person may be reflected in what we know of the main character.
In The Cask of Amontillado by Edgar Allen Poe, the story is written in the first person through the eyes of Montresor, a man looking for revenge on a man named Fortunato. This story has an "unreliable" narrator because we don't personally see or even hear about, Fortunato do anything that would result in someone seeking revenge upon them. Also based upon the dialogue it seems that the two are very good friends so in my opinion the narrator, Montresor, is a lunatic and therefore "unreliable". The story has a combination of descriptive narration where we see into the tricky, deceptive mind of Montresor and dialogue where we see his open relationship with Fortunato. Montresor is a very two faced character and he admits it, "I continued, as was my wont, to smile in his face and he did not perceive that my smile now was at the thought of his immolation." (pg. 263 Poe: Kelly) This is another reason why our main character is "unreliable".
Poe always seems to have psychotic main characters and really goes into the human psyche, deep emotions that are kept deep inside and the external person that the world sees. If one reads a lot of Poe we might better understand the mind of the truly insane.