Winston's perception changes from possible Party overthrow or rebellion in the end of book II to complete and total fear in book III. In the beginning of book III, Winston finds himself in the punishing and sterile environment of the Ministry of Love:
He was in a high-ceilinged windowless cell with walls of glittering white porcelain. Concealed lamps flooded it with cold light, and there was a low, steady humming sound which he supposed had something to do with the air supply. A bench, or shelf, just wide enough to sit on ran round the wall, broken only by the door and, at the end opposite the door, a lavatory pan with no wooden seat. There were four telescreens, one in each wall.
This setting completely blocks out comprehension of the once known to him outside world. This outside world has windows and people and something to look at. This place just has something that looks at him... non-stop.
How would that feel to be constantly watched?
When mixed with other prisoners, Winston notices the described difference between Party and common criminals. People in here just for petty crimes don't seem to have a care in the world. Party criminals wear fear. This further develops and justifies Winston's growing perception that he is in great trouble and he fears the coming tactics whatever they may be. The fact that they are unknown demonstrates part of the philosophy of the rulers of the party. Keeping Winston guessing makes this understanding more unbearable.