I was not discounting Olivia's skill and even agreed she deserved her Oscar...but I think her persona works against her in this kind of story. We know that she represents a more refined type of gal. In THE SNAKE PIT, her character's mental illness is what's keeping her down, but she's not a roughly hewn common woman. In THE HEIRESS, her cultured sophistication is very pronounced. So even if temporarily swayed she must come to her senses and jettison the cad.
With Alice Faye, the character comes across more hard up. This is because of Faye's persona. She plays sweet women who need sex, even bad sex. She's the fallen angel of the story.
I'm with TopBilled on this. Olivia's refinement and elegance were assets, certainly, but not for Fallen Angel. Now, her sister Joan Fontaine, was refined, sure, but Joan had no problem going into the sordid and tawdry world of noir.