This section of the novel occurs at the very beginning of Part II, when Julia deliberately trips up in front of Winston in order to be able to hand him a note. Consider how Winston responds when he sees Julia trip and hurt herself:
A curious emotion stirred in Winston's heart. In front of him was an enemy who was trying to kill him: in front of him, also, was a human creature, perhaps in pain and with a broken bone. Already he had instinctively started forward to help her. In the moment when he had seen her fall on the bandaged arm, it had been as though he felt the pain in his own body.
Winston is presented here as somebody whose natural sympathy and feeling for those around him is so strong that he is controlled by his human emotions. Even though he sees two sides to Julia, viewing her both as an "enemy" and also as a "human being," he is shown in this quote to respond unthinkingly and instinctively, without his normal caution and wariness kicking in. The empathy he feels towards Julia, expressed when he feels the pain himself that Julia experienced, is actually a weakness in this world where everything is watched and scrutinised. Julia's paleness is explained in retrospect when the reader observes how her fear increases the longer Winston tries to check that she is OK. The longer they spend together, the greater the risk, and it is Julia who is presented as being more in control of herself than Winston, who lets his natural goodness dictate his actions. This raises massive questions about his status as a hero. Unfortunately, in the grim dystopian world that Orwell paints for us, to have your actions controlled by your goodness is not heroic; it is merely stupid, and an error that could easily lead to a premature death.