Laurie, horrified by the gruesome, murdered bodies of her friends Lynda and Bob, is too shocked to notice “the outline of a man standing right behind her.” 

Let’s call this the “BEHIND YOU!” Technique. Notice the subtle layering of objects that we see, but that Laurie does not… the outline, the shape, the mask, the knife, the gleam. This creates the effect of slowly increasing the volume of the suspense.