A brief rundown of the plot is in order: We meet Walter Neff (Fred MacMurray), as he literally shuffles into his office. He turns on his Dictaphone; his coat falls back, and we see a hole in his shoulder. He begins to record a message for his boss, Barton Keyes (Edward G. Robinson), regarding a recent insurance claim: "you said it wasn't an accident, check. You said it wasn't suicide, check. You said it was murder... check." And thus begins Walter's "confession" of the murder of his client, Dietrichson (Tom Powers) and his torrid affair with Dietrichson's wife, Phyllis.
The current ending of the film was not the one written by co-author Raymond Chandler. Originally, the film would have ended with Walter's execution in the California gas chamber, but following previews the scene was cut - over Chandler's vocal objections (A scene in which Phyllis and Walter commit suicide was also written, but was rejected and never filmed). The Production Code Administration's Joseph Breen found the execution scene "unduly gruesome", and Wilder did not feel that it matched with his vision of the film, so it was excised from the picture. Though lost, production stills of the scene survive.
blog discusses Stanwyck's concerns with the role - her fascination with the character, and her fear that she - who had never played such a whole-hearted villain before - was not equipped to play the complex part. Luckily, Billy Wilder thought otherwise, and he again was able to get the right actor for the right part. To me, her most interesting scene is the death of her husband, but the second scene that comes to mind when talking about the film is her descent down the staircase, her leg adorned with a gold anklet. RogerEbert.com has an interesting article that discusses the anklet, which was not a part of James M. Cain's original story. The addition of the anklet provides insight into both Phyllis and Neff - Phyllis marked, in a sense as the possession of a man she despises, and Neff, who finds the anklet almost as fascinating as the woman who wears it. The anklet is a sexual bond between the two.