The Apartment
It just occurred to me that Jeff Sheldrake (The Boss) is actually the pivotal character in the script. He is the object that determines the actions of the two principal characters 1) he rekindles his affair with Kubelik and promotes Baxter. Both are defined by their relationship to him and both are motivated by the chance of advancement - to being an executive or being a wife. They are both willing to compromise their principles to achieve their goals. In both cases, Sheldrake is the key - so to speak.
We know Baxter and Kublik are both good people because of the perfect casting. Jack Lemmon - Wilder's favorite actor - is an everyman. He is good, but corrupted by the chance of advancement - as is she. Shirley MacLaine brings an aura of sweetness - she wears a flower - but is also driven to her goal. It's vital we buy into their inner goodness because the denouement is when they both find their "better angels." They both turn their backs on Sheldrake. Lemmon walks - MacLaine runs.
Billy Wilder gave Fred MacMurray his two best roles - here and in "Double Indemnity." Usually cast as the nice guy, both times he is cast against type. I wonder if Wilder's cynicism is at work here; beneath every "nice" guy is a stinker.
The end is very satisfying because Lemmon and Kubelik become the people we know they can be. They don't just find each other - they find themselves.