I hope you all don’t mind if I chime in because I lovelovelove the movie. I haven’t seen the stage shows so I can’t really comment on them but I think Minnelli is brilliant. IMO, the key to Sally in the movie is that she isn’t the world weary jaded chanteuse that she would like to be. She’s actually quite provincial yet pretentious. She’s attracted to Brian because he’s unobtainable. She lives in a world where she needs to be the center of attention, and she sees him as a challenge because he isn’t coming on to her. At the same time, my take is that Brian is attracted to her neediness and vulnerability. I always saw him as moving to Germany to try to escape his sexuality. IMO he would like nothing better than to actually fall for a woman, get married and have a “normal” life. He takes on the role of her caretaker. I don’t see heat between them; I see him caring for and about her, and her desperately needing to be cared for. I’m not sure about the stage versions but the movie clearly draws parallels between Fritz and Brian, who both move to Germany and establish themselves with false facades, Fritz as Christian and Brian as straight (although he admits to Sally that he doesn’t sleep with girls, he is all too eager to state that those were just the “wrong three girls”). In both cases, the conclusion of the movie requires that each acknowledge his “true self”. In Fritz’s case, that means he gains short-term happiness with the woman he loves but with the shadow of the Holocaust over him at the end of the film. Brian, OTOH, loses the girl in the short term, but as the narrator, he presumably goes on to some future success/happiness.
The crux of the story for me is Sally’s abortion. It is at the same time the most selfish and the most unselfish thing that she does. At that moment she suddenly becomes the mature one. While superficially it is a supremely selfish gesture, what Minnelli does brilliantly is to convey that there is more to the decision than she lets on and that it has everything to do with the fact that she sees how wrong it would be for Brian to settle down and marry her. She willingly shoulders all of the responsibility for her decision, even though it means presumably destroying the friendship in order to set Brian free.
Finally, I love the Greek chorus effect of the songs. Not having Brian sing fits with the way the movie is shot, since the songs are commentaries on his life. It’s not particularly subtle, but it’s not meant to be. There is meant to be a obvious juxtaposition between the raunchy lighthearted “Two Ladies” perfomed at the club, and the realities of emotional entanglement actually caused by a romantic triangle in the outside world, or the contrast between “Money” where being rich solves all one’s problems and Maximillion’s world where he doesn’t seem to know what he wants and is constantly searching for the next diversion.