Northanger Abbey is a bildungsroman, a coming-of-age tale in which the heroine or hero sheds his or her naiveté. In the beginning of Northanger Abbey, Catherine does not see the obvious flirtation between her brother James and her friend Isabella, and she does not understand what Isabella is doing by flirting with Frederick Tilney. Catherine has difficulty identifying people's motivations, which, as Henry points out, causes her to assume that people do things for the same unimpeachable reasons she would. As a result, Catherine thinks well of almost everyone, and is frequently too charitable to such people as Isabella and John Thorpe. As the novel progresses, Catherine starts trying to understand people and their motivations, although this pursuit is influenced by her overactive imagination. She attributes General Tilney's grumpiness and odd behavior to guilt over murdering his new wife. After Henry scolds her for this terrible and unfounded suspicion, Catherine comes to a new realization about the nature of people. She understands that people can be both good and bad, because real life is never as black-and-white as it is in the novels she reads.

There are several reasons why Catherine starts to believe that the General killed his wife. The first is that she has just read a Gothic novel, The Mysteries of Udolpho, by Anne Radcliffe, and has come to associate old buildings like Northanger Abbey with the mysterious buildings she encounters in her reading. Catherine arrives at the Abbey feeling that she is in a Gothic novel herself. As she later admits to herself, she arrives at the Abbey "craving to be scared," and when she finds it to be a very boring place, she makes up her own secrets. When Catherine finds out that Mrs. Tilney died of a mysterious illness nine years earlier, and that Eleanor was not there at the time of her mother's death, she feels her suspicions of General Tilney are confirmed. After that, every odd quirk of the General's makes Catherine feel certain that he has a guilty conscience. Her desire to be scared becomes a self-fulfilling prophecy. Soon, Catherine is swept up in a paranoid fantasy, and even entertains the idea that Mrs. Tilney is alive and held captive in a dungeon beneath the Abbey. She does not wonder why the General would murder his wife. She sees him as a cardboard villain from a novel, a purely evil person who would certainly murder his wife without a second thought. Once Henry chastises her for her morbid imaginings, and shows her how illogical her suspicions were, Catherine wakes up from her fantasy and realizes how silly it was. She begins to understand that the General may be gruff and sometimes mean to his children, but he is not evil, and he is not a murderer.

The antagonist of the novel is the character who opposes the protagonist's goals. For most of the novel, General Tilney does his best to make Catherine feel comfortable, because he thinks she is rich and wants her to marry his son, Henry. So to Catherine, the protagonist, he is very pleasant. To his children, the General is alarmingly bossy. He has a generally gruff nature that makes him seem unpleasant. But he does his utmost to make Catherine feel welcome until the end of her stay, when he acts badly by sending her away abruptly, with no explanation. This is the most cruel thing that anyone does to Catherine in the course of the novel. We discover later that the General sent Catherine away because John Thorpe told him that her family had no money. This infuriated the General, who had hoped to marry John into a rich family.

Complicating the matter is the fact that Catherine has imagined the General as a villain from a Gothic horror novel. Since the reader sees the General through Catherine's eyes, the General seems to become a true villain, at least for a few chapters. Even after Catherine realizes her mistake, a lingering doubt about the General and his behavior remains, especially when he sends Catherine home so rudely. Although the General behaves badly, however, he is not indisputably villainous. On one hand, he is greedy, rude to his children, and obsessed with wealth and class. On the other, he is a loving father and capable of being a gracious host to Catherine. An arrogant man like John Thorpe, were he to play a larger part in the novel, could easily become the antagonist. However, no one in the novel actively, constantly works to thwart Catherine or her hopes, which means the novel has no true antagonist.