Foreshadowing can be used as a cudgel to hit the reader over the head and make him pay attention and prepare himself for what is going to happen or it can be a subtle tool that the reader can miss until the thing that was foreshadowed does happen.

In scene 6 of  A Streetcar Named Desire, Blanche turns to Mitch and says about Stanley:  "The first time I laid eyes on him I thought to myself, that man is my executioner!  That man will destroy me. . ."  The reader is put on notice that something serious is going to happen between Stanley and Blanche.