So, I was re-listening to the episode with my wife today. The first thing that struck me was the way Paul and Amy were talking about the characters -- "What's the motivation for the character? Why don't they do this instead? What do you think their relationships are all about?" The second thing that struck me is the way that my wife and I then started having a similar conversations about the movie and the characters. And then, of course, the same thing is happening here on this board.
I think that this, perhaps above all, is why the movie (as well as some of the others on this list) is so good ... we're only with these characters for roughly two hours, and yet this movie is so textually rich that viewers can deep-dive on it without even realizing it. This movie makes it so superbly easy to treat and talk about its characters like they are real people that we know. It's a total character study. Movies like Platoon, King Kong, and Titanic have other things happening, but All About Eve was just ... well, all about Eve, and Margo and Addison, et al, and how Eve's arrival upsets the balance between all of these people.
The writing is so deep, the characters are so realized, and the acting is so good that talking about this movie feels like gossiping about friends.