Winston fears rats. This is first exposed in chapter 4 during one of his escapades with Julia in the room on top of Mr. Charrington’s junk shop. Julia spotted a rat poking its head from a corner of the room and this terrified Winston to the point of screaming. He then revealed that he feared rats more than anything else in the world. This same fear is what O’Brien eventually used to get Winston to betray his love for Julia. Winston had actually intended to keep his love for Julia in his heart forever but upon transfer to room 101 and additional threats of a cage of hungry rats, he betrayed Julia.
Winston also feared being executed for thought crime. During one instance, he encountered three men who were executed for treason which they allegedly committed during a party function when in reality they were in the same venue as he was during the alleged time of the crime. Winston, noting this experience as real history, had kept their photograph but later threw it into memory hole.
Let us not forget the rats! Winston finally betrays Julia in Room 101 because of his horrid fear of rats and the cage containing two rats that O'Brien brings in. O'Brien tells Winston that the cage will fit over Winston's face and that they will devour his face, and Winston screams, "Don't do it to me! Do it to Julia!" AFter which, Winston never feels the same way toward Julia. We find out later that she, too, has betrayed Winston in the same way although we don't know the fear that she was subjected to in order to encourage her betrayal.