I have a theory! *spoilers inside*
I've watched this film several times and the more I watch it the more something develops out of it that I hadn't noticed before. Here is my theory you guys and tell me what you think of it.
I always had this theory that Sissy Spacek represents the childhood of Shelley Duvall's character. If you notice the way Spacek acts, she runs around acting like a little child in the beginning. Playing around on the play area outside that bar just like a little kid would. Swinging around on the wheel chairs at work. The way she plays with her food. Her body language is that of a little girl.
They have the same name Mildred. But Pinky-what Sissy Spacek goes by is a little kid's type nickname and Millie-what Shelley Duvall goes by-is just short for Mildred. But they are the same woman just at different stages of her life.
And that the other mysterious woman was Shelley Duvall's mother and the man was actually her father who sexually abused her when she was a child. That's why Sissy Spacek was totally devastated when Shelley Duvall sleeps with that man (and she falls into the pool with that painting inside). That night represents when the molestation began and after that night, Spacek is changed.
After the night that Duvall sleeps with the man-Spacek is now a more bitter and bitchy and wild girl-to me this represents how the molestation effected her as a young girl.
And there were also some other clues. There's a scene people need to play closer attention to. I remember when Shelley Duvall was walking with one of the old people she was telling them a story about her mother bringing her tomatoes and she doesn't like it. Sissy Spacek later mentions how she can't stand tomatoes. And Duvall mentions she was 11 years old when her mother left her. What does Sissy Spacek act just like? An 11 year old!
And Duvall as a grown woman is delusional and crazy and because she's been molested feels the need to have the attention of men all the time that don't really want her and has a view of herself that is wrong. She wants everyone to think she's so glamourous and well liked and men like her despite the contrary. She is a joke to people around her and doesn't realize it. They see her as the crazy girl. And Duvall is so messed up and insecure she craves adoration. Pinky says things to her like "I think you're perfect" that's what Duvall is trying to tell herself because she is so insecure.
And the night that the woman gives birth to a stillborn baby could have bene something else that happened in Millie's childhood that has got to her. And she slapping Sissy Spacek screaming "you should have went!" to me it seemed as if she was really saying that to herself, because inside she still blames herself.
The ending represents the fantasy of what she wants to happen. Her and her childhood self and her mother get together to kill the abusive father.
So basically what I'm saying is Sissy Spacek's character represents her childhood. And it shows that Shelley Duvall is still haunted by her childhood (Sissy Spacek).