The three act structure is not a story structure. You can't find it in myths and legends or other great stories of the past and you can't find it in nature. So why is it being applied to the screenplay or the story of a film? It's a good question because it makes no sense. And my very strong recommendation in this article will be that you avoid thinking in act structure terms when creating a story or story film.
The three (four, five, six, or seven) act structures are the arbitrary divisions of the principal (or main) action of the story into a number of parts - a legacy from the theatre and applicable today only to the theatre or television shows which have commercial breaks. If you write a movie for television, it will have seven acts. Why? Because it has seven commercial breaks. And you will be asked to insert something intriguing at the end of each act to lure the audience back after the break. But that has nothing to do with story.
The Greeks had no act structure in their plays. The plays had one act. The Romans had five acts. It's arbitrary. It appeared in plays because of the need to have intermissions. People can't sit for three hours in a theatre listening to an auditory experience without taking a break or going to the restroom. It appears in television shows because they want to have commercial breaks so they can sell something. None of which has anything to do with story.
A two hour feature film shown in a movie theatre is a continuous action. There are no intermissions. It's one continuous act-less event which revolves around a problem. A much better way to look at a story, when you are creating one, is not through any arbitrary division into acts but through the eyes of that problem, which is the central event and the heart of a great story's structure.
In The Silence of the Lambs, a serial killer is on the loose, and that is the problem that has to be resolved. In Gladiator, a tyrant has usurped the Roman Empire, preventing the restoration of the Republic. In The Sixth Sense, a murdered child psychologist is stuck in limbo and the spirits of dead people are haunting a little boy's mind. In Independence Day, aliens have invaded the Earth. In Star Wars, the Evil Empire has taken possession of the galaxy. In The Iliad, not to be mistaken for a movie called Troy, the Greek army is being decimated because their best warrior has dropped out of the fight. In King Arthur, the kingdom is in a state of anarchy and has to be reunified. In Harry Potter, Voldemort is trying to take possession of the Wizard World. In The Lord of the Rings, Sauron, a very similar dark force, is trying to take possession of Middle Earth. In Ordinary People, a young boy is suicidal. In The Exorcist, it's a problem of demonic possession. In Jaws it's a shark problem. In The Mummy it's a mummy problem. In The Perfect Storm it's a weather problem. In Jurassic Park it's a dinosaur problem. In Traffic it's a drug problem. In Armageddon it's an asteroid problem. In Erin Brockovich it's an environmental problem. Each of these stories and hundreds of others I could name all revolve around a problem that has to be resolved.
And what need is there to think of these events as having three acts? None.
What use would it be to think in terms of three parts (or acts) when creating a story like A Beautiful Mind - which, if you wanted to divide it into parts, clearly has five parts and not three. In the first part, Russell Crowe is a genius mathematician, in the second part, he is a spy; in the third part we discover the first two parts were a delusion and that he is really mentally ill (the problem); in the fourth part, a first effort is made to solve that problem which fails; and in the fifth part, a second effort is made to solve that problem which succeeds. How would it help to impose a three act structure? It wouldn't.
What good would a three (four or five) act structure do if you were writing a novel - the DaVinci Code, for instance? If you really want to gauge how irrelevant act structure is to a story, try to apply it to a novel. It makes absolutely no sense.
You quickly realize the idea is absurd. It has nothing to do with story. But the screenplay which becomes a story film is a story in the same way that the novel is a story. The spine and structure of both are essentially the same. This is true of the great myths, legends, fairytales, as well as the classics and modern blockbusters. They all have the same basic structure. (See my previous eZine articles: Beyond Theme: Story's New Unified Field (Parts I, II, III); The Essence of Story; and Conquering The High Concept (Parts I & II).
Story has adopted these problem-solving structures from real life. From real serial killers that have to be caught, real terrible diseases that have to be cured, real lost or kidnapped children who have to be found, and real man-eating sharks that have to be destroyed. The principals of dramatic action are the laws of problem solving action in real life artistically treated - and the actions that solve these problems in real life don't contain a three act structure.
So why impose that oddity on a story which is destined to be filmed? Perhaps it's happening because it makes story structure seem simple, which it is not. You can work with the three act structure for twenty years and still not make a story come out right.
What is the alternative? In my opinion, it makes much more sense when you're creating a story to be thinking in terms of the natural structure of the problem which has two main parts: the action that created it and the action that will resolve it. The action that creates the problem is called the inciting action and the action that resolves the problem is called the principle action. The threat, which is the driving force of the inciting action, be that a villain, an asteroid, a shark, etc., is the cause the problem. The anti-threat, which is the driving force of the principal action, be that a protagonist or a hero, is the one who opposes the threat and solves the problem. Either of these actions will acquire the components of the classical structure if there is resistance - which is to say if there is sufficient resistance, there will be complications, a crisis, the need for a climactic action to resolve the crisis, and a resolution.
In Harry Potter, Voldemort is the threat that creates the problem. He is also the main source of resistance that creates the complications and crises, and the need for climactic actions to resolve the crises whenever Harry attempts to solve the problems Voldemort creates. In The Silence of the Lambs, Buffalo Bill is the threat that causes the problem and also the main source of resistance creates the classical structure when Jodie Foster tries to track him down. In The Lord of the Rings, Sauron is the threat that is causing the problem and is also the main source of resistance that creates the complications, crises, etc. when Frodo and his little Fellowship try to solve the problem by destroying the Ring of Power.
Aristotle's classical structure, which is the dominant feature of this structure, can stand alone. All of the structures you might find in the act are already built into the problem solving action that encounters resistance, namely: conflict, complications, crises (turning points) climax and resolution. It is, in fact, the structure of any problem solving action (real or fiction) that encounters resistance. From there, the natural thing to do is divide the principal, problem solving action into scenes, which are the ideal units of action to reveal these larger, essential actions.
After the story is created, of course, you can divide the action into any number of parts that you like, but it's counterproductive to think in those terms at the story's inception. In other words, you shouldn't be using act structure to lay out or create the story.
However, if you need to use the three act structure because you're pitching an idea to someone who only speaks that language, then follow Aristotle and translate the idea of three acts into a beginning, a middle, and an end and you'll be able to communicate with them. Then, if you're asked: what is the first act? Tell them how the story begins (which is really what they want to know) and make it as intriguing as possible. If asked: what is the second act? Tell them what's happening in the middle of the story (which includes the main crisis of the dominant plot) and make it as stressful as possible. If asked about the third act, tell them what the climax of the story is (and make that as exciting as possible) - and finally how the story is resolved - and make that as satisfying as possible.
To conclude, what I'm saying is this: when you're creating a story, you should put aside the archaic notion of three acts and focus on the natural structures surrounding the problem, which is the central event and heart of your story.