Syd Field, author of Screenplay and The Screen Writer's Workbook, has outlined a paradigm that most screenplays follow. A paradigm is a conceptual scheme. This paradigm is the structure that holds screenplays together. According to Field, screenplays follow a three-act structure, meaning the standard screenplay can be divided into three parts: Setup, Confrontation, and Resolution.
Act I comprises the first quarter of the screenplay. (For a two hour movie, Act I would last approximately 30 minutes.)
Act II comprises the next two quarters of the film. (For a two hour movie, Act II would last approximately 60 minutes.)
Act III comprises the final quarter of the film. (For a two hour movie, Act III would be the final 30 minutes.)
The "Plot Point"--According to Field, the three acts are separated by two plot points. A plot point, often called a reversal, is an event that thrusts the plot in a new direction, leading into a new act of the screenplay. Later screenplay gurus have built on Field's theory by stating that Plot Point #1, which leads into Act II, is the moment when the hero takes on the problem.
The Three-act Paradigm: