I will say that Passion is a difficult work to understand, even as far as Sondheim musicals go. But I'm starting to "get" it. The story is about a handsome captain Giorgio who is having an affair with a married woman named Clara. He gets transferred to another station where he encounters the cousin of his superior officer, a sickly woman named Fosca. In the DVD commentary, Sondheim says that Donna Murphy was so good in her audition for Fosca, that she understood every nuance. Murphy says that it's because she was like Fosca a long time ago; she was sick and at some point, her illness started to define her.
"It actually had to do with a period of my life where I was physically vulnerable and was dealing with an illness, a condition that I came to sort of identify myself by that, in terms of that, I should say, which I completely shifted my sense of self. And there was a kind of defensiveness that developed in me..."
I've been going through an "illness", shall we say, and I'm not sure that even I understand completely. All I can say is that once illness takes over your life and becomes your identity, it's very hard to shake.