Somewhere in the first five minutes of a well-structured screenplay , someone (usually not the main character) will pose a question or make a statement (usually to the main character) that is the theme of the movie. "Be careful what you wish for," this person will say or "Pride goeth before a fall" or "Family is more important than money." It won't be this obvious, it will be conversational, an offhand remark that the main character doesn't quite get at the moment — but which will have far-reaching and meaningful impact later.
This statement is the movie's thematic premise.
In many ways a good screenplay is an argument posed by the screenwriter, the pros and cons of living a particular kind of life, or pursuing a particular goal. Is a behavior, dream, or goal worth it? Or is it false? What is more important, wealth or happiness?