In Double Indemnity we see the classic portrayal of a femme fatale. Phyllis Dietrichson is the definition of a femme fatale. She gets Neff to help her through seduction and uses him. She is a very one-dimentional character. We only see her "bad" side where she is trying to kill off characters.
This is in stark contrast to the woman in L.A. Confidential, Lynn Bracken. Though sometimes classified as a femme fatale, she is not using Bud White or trying to harm him. Unlike Dietrichson, Bracken is a complex character. We learn about her childhood and see her room. We see her break down and cry. We see her fall in love. As an audience, we empathize with her. We see multiple layers of her that we never see with Dietrichson.
The images above describe these two women and their relationships. The first is of Lynn Bracken and Bud White. In this shot, we see emotion. Bud is looking lovingly at Lynn, clearly expressing how he feels about her. She, though looking off into the distance, looks very content. They look very comfortable with one another and safe. The image doesn't give you the impression the Bracken is using White or trying to hurt him.
The lower image is of Phyllis Dietrichson and Walter Neff. This photo is much harsher. Neither looks the slightest bit happy. Dietrichson with the jet black sunglasses looks cold and unapproachable. She does not seem to be looking at Neff at all. Neff's eyes are slightly turned towards Dietrichson. This shows that she is in control. He is almost looking to her waiting for her next move. We can really see her power as the femme fatale from this image. She is drawing Neff in, but clearly wants nothing to do with him. She uses him.


