Four Act Structure
The four acts in this structure have acquired various names from different analysts, but in this discussion I'll refer to them as the Set-up, Response, Attack, and Resolution.
This structure is essentially the same as a three act structure in which the second act, which is lengthy and sometimes difficult to handle all in one chunk, has been broken into two more manageable parts. This produces four acts of roughly equal length.
ACT 1: SET-UP
As in the three act structure, the first act here functions as an orientation to the story. Act 1 should accomplish the following:
establish the genre and tone of the narrative
ground the reader in the world of the story
introduce and create empathy with the main character
introduce the force of opposition (though it may remain mysterious at this point)
establish the stakes of the story
provide context for the meaning of the story as a whole.
In the four act structure, act one ends with an inciting incident, an event, decision, or realization that thrusts the main character into an overwhelming conflict, beyond the point of no return.
ACT 2: RESPONSE
In the second act, the main character reacts to the inciting incident. Under attack and on the defensive, the character struggles to survive, to understand, to make preparations, to gather allies. Obstacles arise. Complications abound. Enemies are around every corner. More is revealed about the character's backstory, goals, needs, and motivations. The stakes rise. Drama builds. The tension increases.
Act 2 ends with another event, realization, or decision that, again, changes everything, empowering the main character to stop merely reacting, and to go on the offensive.
ACT 3: ATTACK
In the third act, the main character shifts into a proactive mode. Instead of just reacting to what's happening, the character takes the fight to his opponent. A plan is made, assets are gathered, and the character begins to push actively toward a satisfactory resolution.
But it's not enough! The opponent has also been gathering strength and pushes back with even greater force than before. The main character needs more. More courage. More resources. A better plan. A mightier weapon. And the enemy just keeps on coming. It seems as though all is lost.
Act 3 ends with another game-changing event, realization, or decision. This time, the main character acquires the last thing needed in order to triumph. The missing information. The last piece of the puzzle. But the tools on the table at this point are all there is to work with. No more help is coming. Time is up. The final conflict is at hand.
ACT 4: RESOLUTION
In Act 4, the tension reaches the breaking point. The main character launches into battle one last time and either triumphs decisively at last, or is completely and irrevocably defeated. The effects of the final outcome of this confrontation--the changes wrought by the story--are then shown to the reader, and the story comes to a close.