Having never fully understood the concept of "film noir" this is a great place to start!
Although I have never seen the film, the opening sequence indeed sets the tone of the film. Much like others have said you can feel the heaviness and darkness of the film. At first I thought the children were in a school yard, only to have the camera pan up and out to show an apartment complex. Even with the lady yelling at the children, later you see that she still has some concern for their well being, which you later discovery that concern was well warranted. The use of shadows and character activit