Winston is thoughtful, inquisitive, and reflective, while Julia is more passionate and sexual. Even at the beginning of the novel, Winston seems to have an active inner life where he thinks critically about the society in which he lives, how it functions, and how it came to be. He takes pains to keep these thoughts and ponderings completely concealed, of course, but his desire for intellectual freedom is present from the opening pages. To Winston, the greatest act of rebellion is finding ways to have and express free thought in a world in which free thinking is illegal and dangerous. He is fascinated by "The Book," which lays out the history of the world's super-states, as well as how they use propaganda and terror to stifle intellectual and personal freedom. He finds life in Oceania intolerable because of his inability to show authentic thought or emotion, and the cognitive dissonance of being forced to believe information that he knows (from his job at the Ministry) is absolutely false.
By contrast, Julia's strongest desire is to be able to express sexual love. For her, the most onerous aspect of life in her society is forced abstinence and the reduction of sexual intercourse to a purely procreative activity. She indulges Winston's desire to visit O'Brien and read "The Book," but she is not nearly as curious as Winston to understand the inner workings of their society, nor is she as committed to toppling it. Her primary expression of rebellion is to seek and obtain sexual gratification. This, perhaps, is what prompts Winston to describe her as "a rebel from the waist down."