Three-Act Structure

Hollywood movies use a very well defined, three-act structure. In fact, it’s so well defined, you can usually set your watch to it.
Act I: The beginning.Wherein the main character is introduced, along with his or her nemesis, and the supporting cast. The main character’s “problem” is also introduced. This section usually takes 20 to 30 minutes.
Act II: The middle.The main thing that happens in Act II is that the character’s  problem is complicated. This is the bulk of the movie, and any subplots are usually introduced and solved here. Very rarely are new characters introduced in this act. Halfway through the act—that is, in the exact middle of the movie—the main character’s luck will change, and things will start working out in his or her favor. By the end of the second act, the solution to the problem will be clear. This act is usually 40 to 50 minutes long.
Act III: The end. In this act, the main character’s problem is solved. This usually takes about 20 minutes.

Yet another way to define this structure is: Act I: Introduce the hero; Act II: Torture the hero; Act III: Save the hero.
       Unlike a stage play, there is never a clear separation of acts presented in a movie. The actual change might be a simple cut from one scene to another, or the scene might just barrel on ahead into the new act. It’s not important that the audience knowswhere the act breaks are, but that doesn’t mean that they aren’t there.
You don’t have to follow this structure, and many movies don’t. However, it is a fairly simple, effective way of ensuring a compelling story that “moves.”