If you look closely in one of the shots looking at the musician's pad you can see Hitchcock making a cameo like he does in all his movies. If you pay attention however, you might notice that this is a reenactment of a scene from one of his other movies, 'Rope.' Also, Hitchcock is standing in for James Stewart and the character he played in 'Rope' since Mr. Stewart is busy spying on this scene.