As Julia and Winston develop a physical relationship, which components of their liaison influence either character, or change them to a certain extent?

While the relationship between Julia and Winston begins as a purely sexual liaison, it soon develops into a mutually caring relationship. The two fall in love with each other. The first component of the relationship that begins to change them is, therefore, the genuine love they experience. Winston, especially, starts to become more humane. He moves from having sexual fantasies of harming Julia to wanting to protect and care for her. Both characters are willing to sacrifice themselves for the other.
A second component that influences them is having the room they can escape to over Mr. Carrington's shop. This offers them something like what was, in earlier times, a normal domestic life together. They lie in bed with each other, drink coffee together, and talk and dream together. This experience whets their desire for a life more fulfilling than the one they currently experience as outer Party members.
A third component that influences them is the knowledge they both have that their situation can't last forever. This encourages them to make the most of the fleeting time they do have, and it also encourages them to seek out what Winston believes is a secret, revolutionary underground plotting to overthrow the state.
The most important component of their being together is the relationship they build, which matters more to them than anything else. This is what is an intolerable thought crime in the eyes of the government.