While many prefer using a body double for such a scene where their behinds would be on full display, Aniston did not.
In an interview, Aniston revealed that she “definitely didn’t” use a body double to film the nude scene.

Why? Well, because it would have been pretty difficult to switch between the two because of the way it was to be shot.
According to her, it would have been quite difficult to alternate between the two because of the way it was going to be shot:
“The problem is because the way it was shot, it was so specifically me… They wanted to make it a tricky camera so you saw the face and then you go to the back. It was hard to cut, unless you did it,” Jennifer Aniston explains. However, her decision didn’t turn out to be entirely favorable, as after the film’s release, discarded shots from the shoot began to circulate on the Internet, showing more of her body than what was seen in the final cut.