I love Joan Crawford (can't stop thinking about MP) but her performance is 180 degrees off in that movie. It's part of why it works--if she were just a little bit off, then we'd notice more--and part of why it matches in theory. One gets the feeling Crawford couldn't see her character's fatal flaw quite as a flaw--or she rarely acts it as such, I should say--since it was about the maternal instinct gone awry (easy to imagine her wanting to hide psychologically there herself, if we are to believe Things). So her reactions are all within the spectrum of good, noble--human, but good. She just sometimes boils over, but never really loses it. She's all modulated Joan.
Perhaps it's the sense that it is face-slapping Crawford inhabiting this role with all the potential energy and rage it carries that gives the edge to her performance--that encourages us to bring something to our viewing of her performance. Maybe it's the combo that works. God knows she slaps faces in this movie too, but it's not crazy Joan. Whatever the appeal of the performance is, it's sort of oblique. She inhabits her sideways.