Julia notices a rat running across the room. She throws a shoe at it, seemingly unaffected by the vast presence of rats throughout London. She seems unfazed by the rat's presence, which is not what one can say about Winston. He is positively petrified by the presence of the rat. It triggers so much in way of fear and negative thought that he simply cannot restrain himself:
For several moments he had had the feeling of being back in a nightmare which had recurred from time to time throughout his life. It was always very much the same. He was standing in front of a wall of darkness, and on the other side of it there was something unendurable, something too dreadful to be faced.
Even the "warmth of reassurance" from Julia's body cannot soothe Winston's intense reaction to the rat and his sense of absolute fear. It becomes telling that Julia cannot ease Winston's pain of rats, something that is used later on in the narrative to a brutally sad end.