I think Miriam Hopkins steals the show, here...
I think Bette Davis is a fantastic actress, and she gives a very secure performance here. The fact is, though, her character isn't all that interesting. And Davis is creamy, poised, smooth, intelligent, etc., as Kit, but she doesn't do anything here she doesn't do in other movies. Plus, it's kind of hard to feel bad for her character at all, when she's been pushing guys she loves away from her for the entire story.
Hopkins, however, is a hoot! What a f-ing infuriating firecracker! I don't mean to imply she's a BETTER actress than Davis...but in this movie she's a lot more fun to watch!
It's interesting that the two seem to be in different movies - Davis is playing a sincere drama while Hopkins is playing for comedy. But at the same time, they're completely believable as friends. (It also says something that Davis was actually a fan of Hopkins' work, despite their personality clash.)
Maybe part of the problem is, for me, it's hard to appreciate these movies as they were intended for their original audiences. But to modern eyes, for these 2 hours, anyway, I think Hopkins is a LOT more fun to watch.
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EDIT: Oh! I just listened to Vincent Sherman's audio commentary and he feels the same way!
VINCENT SHERMAN: "[Hopkins] knew exactly what she was doing. And there's no question now that you look at the picture, Miriam really steals the picture in so far as performance is concerned. Bette gives an honest performance and is very dignified about it and gives a very good performance of a very difficult and not a colorful role. But Miriam, colorful...Miriam's role was very colorful and she took advantage of it. In fact, an old friend of mine, Kazan, was out at the studio one day and Blankey" [?] "wanted to show him some rushes of what I was doing with the girls, and he said to me afterwards, he said 'How did you get Miriam to do all those things? My god, she's wonderful!' But, you know, she must have known that I...I said, 'I just told her what to do and she did it.' Well, she knew that I was staging it for her, to become the temperamental, completely different character. I had been watching them, you know, as they walked around in the scene and I said, 'Well, move where you feel like moving, and we'll go on from there.' And I didn't want to impose moves unless they came from something inside, what the actors were playing. Well, that was a mistake, because Miriam immediately, you know, began inventing reasons why the camera should always be on her and not on Bette! When I said something about it, Miriam said, 'Oh, well I was only trying to cooperate, you see.' She was cooperating, alright!"