One of the principal difficulties in a scenario like that of Rear Window, I imagine, was to make the spectators complicit, during the entire film, with the pure voyeurism of the protagonists. This would have been difficult because, except at the end, they are not threatened by the (possible) murderer whose actions they study, nor is it necessary for them to defend anyone, since the crime has already been committed by the time they start to become interested in him.
However, this difficulty is directly confronted at the very beginning of the film, in the first scene between Jeff (James Stewart) and his nurse, Stella (Thelma Ritter). Stella anticipates the spectator's reaction, with an unambiguous condemnation: “We've become a race of Peeping Toms.” This condemnation is, in fact, something of an endorsement of voyeurism; Stella herself will be, when the moment arrives, the most enthusiastic participant in this sport and will display the most fertile of morbid fantasies concerning the murder across the way.
On the other hand, there indeed is something that is never said or alluded to throughout the film, and that must not be – because the whole working out of the story depends on its repression.