Since The Bell Jar is told from the intensely personal point of view of Esther Greenwood, thoughts and opinions are a major way to access what makes Esther tick. The novel is also, in its own way, a novel of ideas: ideas about women, sexuality, literature, mental illness, and death, to name just a few of the big ones. If characters other than Esther seem a little flat, it's not only because we never get their point of view, but also because much of the time they stand for a particular idea. Thus Mrs. Willard, for example, gets to be the voice for traditional roles for women as wives and homemakers.
Since the novel is told from Esther's point of view, the only way we get to know the other characters is through speech and dialogue. Many of the important events in the novel are actually just significant conversations. For example, when Esther thinks back over her relationship with Buddy, she mainly recollects all of the disparaging comments he made about her writing or the stuff about women that he quotes from his mother. And since we are in Esther's head, we can best gauge the way people affect her by the way the things they say get stuck in her head, what she calls "my little chorus of voices" (12.65). These scraps of remembered lines – from Jay Cee, Buddy, her mother, a former professor – all contribute to her psychological deterioration and her eventual attempt at suicide.
Esther tells us that she sees the world "divided up into people who had sex and people who hadn't" (7.50), and the novel is formed along these lines because sex and love are two of Esther's primary obsessions. The novel is critical of a society where women are expected to be "pure" and chaste before marriage in contrast to men, whose sexual appetites are matter-of-factly indulged. Esther idolizes characters who seem comfortable with their own sexuality, such as Doreen and even Constantin, even though he and Esther never have any physical contact. But Esther is ruthless toward characters who hold a double standard for men and women when it comes to sex, particularly male characters such as Buddy and Marco.
One contributing factor to Esther's depression is the fact that she's hyper-aware of the way other people think of her. In fact, she seems more sensitive to social and class distinctions than the people around her. As someone who grew up in modest circumstances and had to earn her scholarship to college, Esther seems almost star struck by the life of luxury and comfort of the other interns and the girls at her college, and she's always commenting on the difference between her upbringing and theirs. At a banquet luncheon at Ladies' Day, for example, she brings up the fact that avocadoes are a luxury that she's only tasted because her grandfather worked as a maitre d' at a fancy Boston country club. Esther's sensitivity to social differences seems to be due to the fact that she feels enormous pressure to be a model of upward mobility, a shining example of one modest girl who, through hard work and a little talent, becomes a huge success.