While both Rushworth and Mrs. Norris are funny in their pomposity and their lack of intelligence, their place in the plot is deadly serious. Rushworth represents the negative side of marrying for money; he is dreadful to know and completely incompatible with Maria. His buffoonery is fun, but it will lead to disaster when Maria just can't take it any more. Mrs. Norris is a deliberate caricature. Her hysterical insistence on preserving social distinctions is responsible for much of the mistreatment Fanny receives. In both cases, though, the comedy can distract from the plot a bit. Austen creates such rich and entertaining characters, however, that the distraction is easy to forgive. Their idiocy makes both Rushworth and Mrs. Norris realistic and memorable.
The end of each of the first two volumes finds Fanny in peril, first of being forced to take a part in the play, then of being forced to marry Henry. Austen uses the three-volume form to structure her novel like a play: each set of actions is complete and concludes dramatically. The three volumes are broken up further into a series of episodes. As we progress from episode to episode, lessons are learned, characters change, and the structure of the novel gives us a noticeable sense of movement and climax.
Many of the marriages in this novel are based on money and social position. In this society, marriage is one of the few ways a woman can change her status. Austen depicts many of her characters as caught up in either money or extremes of emotion. The ideal marriage, in this world, is one based on companionship. Despite their flaws, Sir Thomas and Lady Bertram truly seem to value each other's company. The ideal mate for someone like Fanny would be a man with whom she is intellectually and emotionally close without physical attraction playing too excessive a part. Her brother William is a kind of unattainable ideal in this regard. Edmund is perfect, since he is like a brother in his manner toward her but not actually her brother.