Why did the ghost appear in a dark room?
In "The Canterville Ghost," the ghost almost always appears in a dark room. We see this in Chapter Two when the ghost makes his first appearance before the family. He waits until all of the lights have been turned off before he rattles his chains along the corridor.
Arguably, the ghost appears in a dark room because he wants to maximise his chances of scaring the Otis family. By appearing at night and in the dark, the ghost has a better chance of frightening the new residents of Canterville Chase which is, in fact, the reason for his existence, as is made clear in Chapter Four:
"It was his solemn duty to appear in the corridor once a week, and to gibber from the large oriel window on the first and third Wednesdays in every month."
Ultimately, however, the ghost fails to frighten the family, despite his many appearances in the darkest recesses of the house. In an ironic twist, the ghost becomes the terrified victim of the Otis family and decides to leave the house in Chapter Five so that he can rest eternally in the Garden of Death.