James M. Cain, tough guy novelist & screenwriter, was interviewed before his death in 1977.
Many of his great works were made into movies, ex. "Double Indemnity" & "The Postman Always Rings Twice." His novel, "Mildred Pierce", was recently remade for HBO with Kate Winslet.
I thought his words re: the difference between theme vs. plot were so helpful. It is in the 3rd to last paragraph.
Here's an excerpt from the interview:
[I've changed the paragraphing for your viewing pleasure. All parentheticals are mine.]
Q: What are the events behind Mildred Pierce?
A: Jim McGuinness, my old producer friend at Columbia, once made a remark that led to "Mildred Pierce." Out in Hollywood all they talk about is story - secretaries, everybody - story.
Well, one day we were going to lunch, talking about stories, when he said, "There's one that's never failed yet, and that's the story of a woman who uses men to gain her ends." I though, well, if it's never failed yet that sounds like a pretty good story to me.
Secretly, then, I began to try to adjust this formula. For a while I had the woman as an airline stewardess. Then she was a girl who won a beauty contest & came to Hollywood. Neither of these came to life, so I thought, maybe it makes some difference what ends.
I suddenly thought it might help if her children were the ends she used men for, & naturally it would be better if it focused on one child. But I had to have another child in there so it wouldn't seem so pat & easy.
Then made her not a femme fatale at all, just a housewife, but she had that instinct to use men.
Every time I had trouble with that book I thought, "My friend, you've forgotten what your story is about. This is not the story of a woman who is devoted to her daughter & is nuts about her [plot]; it's a story about a woman who uses men to gain her ends [theme]."
Every time I'd remember that & reinstate that theme in the book, it would go.
No reviewer or anybody who read it ever detected that that's what the book is about. I didn't highlight it enough. I don't take much pride in "Mildred Pierce," I have to confess to you. It's not my kind of book. I made some egregious mistakes in it, especially right at the end, which is a very costly place to make mistakes.
From "Backstory:Interviews with Screenwriters of Hollywood's Golden Age," by Pat McGilligan (1986).