It's known in 1984 that Room 101 contains your worst fears; it's the ultimate torture/emotional breakdown technique. However, assuming each prisoner has a different fear, Room 101 would have to continuously be modified specifically for each new prisoner.
How does the Party get around this?
In the novel it seems that a few people each day are sent to Room 101 (that could be a false recollection, though). I doubt the room could be redone that quickly.
Are there several Room 101s?
Is there a whole group of people working on remodeling the room for each new prisoner?