Interesting quote that talks about Maxwell Anderson when asked for advice to new playwrights. This also applies to writers of fiction and the quote is something to remember when writing. "The main character must learn something new about himself before the end of the play, and this must be something that would change his life forever."
What a great idea. Sometimes when I'm writing I find the main character in a story hasn't done this and the story falls flat. That's when I must go back to look at what the character has learned. This doesn't have to be an earth-shattering lesson. Could be just a small thing that has changed the character's life, for better or worse. Since I often write romances, the main character usually learns a bit about trusting other people, or he or she can learn the change has shown them where their life has gone off track.
So not only look at the changes in your characters' lives, but in what the change has taught them. May not take more than a line. I've found this a good lesson when writing a series character, one who is slowly evolving while murderers enter her life and bring small changes.