The Three Act Structure

In Hollywood films the narrative tends to be organized in what is called the 'three act structure'.
The writer Sid Field has identified what he calls the ideal Paradigm three act structure.

In this structure, a film must be set up within the first 20-30 minutes before the main characters or protagonist experiences a 'plot point' that gives him or her a goal that must be achieved.

Approximately half the movies running time must be taken up with the character's struggle to achieve his of her goal: this is the confirmation period.

Field also refers sometimes to the midpoint, a more subtle turning point that happens in Act II - the confirmation which often had as an apparently devastating reversal of the main characters fortune.

The final quarter of the film (the Third Act) shows a struggle by the protagonist to finally achieve (or not achieve) his or her goal and the aftermath of this struggle.