Surely one of the most sublime moments in all of film history, this is also a great instance of a character realising they have been under scrutiny. The big moment comes when Lars Thorwald (played by Raymond Burr) realizes that Jeff (James Stewart) has been watching him. As he realizes this, Hitchcock shows this so beautifully by having the character look directly into the camera (at us), thereby producing a truly shocking moment, all because our look is returned. This is also so great for being done literally without fanfare, the only music continues to be the sound of the piano coming from another department.
Jeff, a photographer, spends most of the movie spying on his neighbors, either photographing or watching them through binoculars.