Even before the narrator arrives at the house, he is struck by the odd tone of the letter.
When he first see Roderick, he notes that is friend is greatly changed, and displays "ghastly pallor" of skin and miraculously lustrous eyes. Ropderick is also subject to mood swings of the sort we would now suspect of being bipolar.
Next, Roderick develops unnatural sensitivity to sound. Roderick's paintings and poetry display increasing traces of his divergence from normality and fascination with the morbid.
The burial of the sister in the house is more evidence of his deterioration.
The next episode is the "wild hilarity" on the stairs. Also, Roderick says that he has been hearing his sister trying to break out of the tomb for days, but he neither investigates nor rescues her.