Although it is not something that the reader readily notices, Usher does change his attitude and mood many times throughout the story as he speaks to the narrator especially when the two first meet, when Madeline passes away, and when Madeline rises again.  First, when the narrator meets Usher for the first time after many years, Usher's voice is described as having "varied rapidly from a tremulous indecision."  However, it is in the very next paragraph that Usher changes his mood and attitude entirely:

It was thus that he spoke of the object of my visit, of his earnest desire to see me, and of the solace he expected me to afford him.

Suddenly Usher is no longer voicing his "tremulous indecision" and is speaking with great decision, indeed.  Second, when Madeline dies, Usher "informed me abruptly that the lady Madeline was no more," but then goes on to extensively explain the "wheres" and "whys" of temporary entombment as well as the request for help with the process.  Again, a major change in mood to move from being blunt to being extensive.  Finally, Usher's countenance changes vastly within his last conversation with the narrator.  When the narrator first sees Usher in this instance:

His eyes were bent fixedly before him, and throughout his whole countenance there reigned a stony rigidity.

However, Usher builds to an unbelievable creshendo as he reveals to "have put her living in the tomb!"  This is an incredible difference from the "stony rigidity" and silence reported in just the paragraph before and marks the most astounding difference of attitude and mood as Usher speaks with the narrator.  What is important to note is that these instances of mood change indicate that Usher does not simply have a physical malady.  Instead, Usher has a very serious and, as we eventually see, very deadly mental malady as well.