But neither the business alleged, nor the magnificent compliment, could win Catherine from thinking that some very different object must occasion so serious a delay of proper repose
something was to be done which could be done only while the household slept; and the probability that Mrs. Tilney yet lived, shut up for causes unknown, and receiving from the pitiless hands of her husband a nightly supply of coarse food, was the conclusion which necessarily followed.
This passage, from Volume II, Chapter VIII, is important for two reasons. First, it is an example of Austen's technique of free indirect discourse, a technique by which Austen narrates the story in a tone reflecting what Catherine is thinking or feeling. Here, the narrator conveys Catherine's certainty that something sinister is afoot. Although the narrator describes Catherine's suspicions with an almost straight face, we can tell the narrator thinks them totally unfounded. Free indirect discourse is similar to the first person perspective, but it is not as limiting. The narrator, while conveying Catherine's feelings, is free to describe things differently than Catherine might, impart information that Catherine does not know, or signal her own opinion about Catherine's thoughts.
The passage is also important because it shows how much Catherine has become the victim of her own paranoid fantasy. She came to Northanger Abbey, as she admits later to herself, longing to be scared, and when there she finds nothing scary there, she must invent something herself.