The portion of this passage that indicates that the narrator observed Roderick Usher trying to appear cheerful and friendly, in contrast to his "terribly altered' appearance is; Usher appeared to the narrator to be making a 'constrained effort."
An individual who makes a constrained effort is one that does something under pressure.
This means that he is not doing it voluntarily, rather he does it because he wants to prove a point or appear a certain way.
The same can be said of Roderick Usher who can be said to be under some constraints. He tried to appear cheerful even though his appearance was terribly altered.
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