Winston is just barely coming to a realization of his hatred for the Party, and is filled with terror and unease in regards to being discovered. He hates the party, has vague suspicions about its honesty and intentions, but isn't quite sure how to define them or go about acting on his hatred. He wants to find others who feel the same, but has no idea how to do so. He wants to discover just exactly how the Party is no good, but can't figure out how he would get that information. So, Winston's feelings against the Party are newer, more unformed and unspecific, confused, and without outlet. He fumbles about, with the diary, and the junk shop, trying, in his own way, to register his rebellion, but it's nothing huge or active. He is also more reticent to participate in mandatory Party activities; he's always on the fringe, hesitant and questioning.
Julia on the other hand, also hates the Party, but is well-practiced at rebelling, and well-versed in why she hates it. She understands its dishonesty more clearly, has formed and strong opinions about its intents, and a more stratgized plan for surviving and yet rebelling at the same time. She has had nearly a decade of acts of rebellion; it's an art form to her. She also doesn't have the same conflict inside in performing Party activities; to her, it's just something that has to be done to survive, and the better you do it, the better your odds are. She yells and serves and exercises with vigor and enthusiasm, all as a ruse to cover her real intentions. Winston isn't so enthusiastic; it's harder for him to filter out his hatred. Julia has processed all of the Party's lies and come up with reasons to reject them all, whereas Winston hasn't developed the ability to do that yet.
I hope that those thoughts help a bit; good luck!