Final answer:
The narrator is most startled by Usher's ghostly appearance and his bizarre combination of excitement and rigidity in his behavior.
Step-by-step explanation:
When the narrator reunites with his childhood friend Roderick Usher, he is startled by Usher's ghostly pallor and the luminous intensity of his eyes. Usher's behavior also evokes anxiety; he displays a strange combination of hysterical energy and a stony, rigid countenance. The drastic change in Usher's appearance and demeanor, compared to the narrator's memories of him, creates an overwhelming sense of awe and unease.