Act 1 or The Beginning:
In a typical feature length screenplay, it is approximately 120 pages long. 1 page = 1 minute of screen time. The first 1/3 is the setup and in a feature you have approximately 30 pages to set up your story. However, there is a point in time where the audience (either consciously or not) decides if they like the film or not. This is usually in the first 10 min, which is 10 minutes of screen play. You've got to hook the reader immediately.
So you have 10 pages (shorter length in short films) to let the audience know WHO your main character is, WHAT the premise of the story is, and WHAT the situation is.
Plot Point 1:
An incident or event that hooks into the story and spins it around into another direction. It can also be an object of significant importance, around which the plot revolves.
Act II or The Confrontation:
Once you find out the need of your character - what he/she wants to achieve during the screenplay, what the goals are - you then create obstacles to that need. This generates conflict.
Everything you do is held together by the dramatic context of confrontation.
Drama is conflict. Without conflict there is no action. Without action, no character. Without character there is no story.
Plot Point 2:
An event that leads to the resolution of the story. This is the breaking point of the character after which nothing is the same. The character changes.
Act III: The Resolution
How does it end? What happens to the main character? Live or die? Succeed or fail? A strong ending resolves your story in order to make it comprehensible and complete.
Now, we've all seen films that are formulaic and I know you are thinking to yourself that if you follow these ideas that's what you'll end up with. It's not that cut and dry. Go and watch your favouite film and see how closely it follows these basic principles and see where it diverts.
Just for fun, here is an example:
American History X -
Act 1 - Set up of what has happened - The murder, Derek in jail.
Plot Point 1 - Derek gets out of jail. The world changes for all the characters and they have to deal with the elephant in the room - Their family member being a murderer and how he has changed.
Act II - Derek tries to deal with being changed - not racist. And deals with Danny being just like how he was. Danny tries to deal with Derek being changed and betraying his beliefs.
Plot Point 2 - The Rape & Derek’s explanation of how and why he changed.
Act III - Danny abandons his beliefs by taking down his racist posters. The conclusion is he is killed.
In his book Screenplay, Syd Field added onto the standard 3 Act Structure and broke up act 2 into two different parts, 2a and 2b. The following is a basic breakdown...
Opening Image: The first image in the screenplay should summarize the entire film, especially its tone. Often, writers go back and redo this as the last thing before submitting the script.
Inciting Incident: Also called the catalyst, this is the point in the story when the Protagonist encounters the problem that will change their life. This is when the detective is assigned the case, where Boy meets Girl, and where the Comic Hero gets fired from his cushy job, forcing him into comic circumstances.
Plot Point 1: The last scene in Act One, Turning Point One is a surprising development that radically changes the Protagonist's life, and forces him to confront the Opponent.
Pinch 1: A reminder scene at about 3/8 the way through the script (halfway through Act 2a) that brings up the central conflict of the drama, reminding us of the overall conflict.
Midpoint: An important scene in the middle of the script, often a reversal of fortune or revelation that changes the direction of the story. Field suggests that driving the story towards the Midpoint keeps the second act from sagging.
Pinch 2: Another reminder scene about 5/8 through the script (halfway through Act 2b) that is somehow linked to Pinch 1 in reminding the audience about the central conflict.
Plot Point 2: A dramatic reversal that ends Act 2 and begins Act 3, which is about confrontation and resolution. Sometimes Turning Point Two is the moment when the Hero has had enough and is finally going to face the Opponent. Sometimes it's the low-point for the Hero, and he must bounce back to overcome the odds in Act 3.
Showdown: About midway through Act 3, the Protagonist will confront the Main Problem of the story and either overcomes it, or come to a tragic end.
Resolution: The issues of the story are resolved.
Tag: An epilogue, tying up the loose ends of the story, giving the audience closure. This is also known as denouement. In general, films in recent decades have had longer denouements than films made in the 1970s or earlier.