First let me say that there are exceptions to everything, but for the most part, the "midpoint mirror" applies to a huge majority of films, especially genre films like Horror, Action, Sci-Fi, etc.
But what exactly is the mirror? Simply put, it’s a reflection, and in story terms, the midpoint reflects – or mirrors – the end of the film, meaning that if the hero wins in the end, the midpoint is also a victory, and if it's a tragic end, your protagonist will reach a low point half way through.
This ‘mirror’ structure allows the story to move up and down, all dictated from the end: (1) high midpoint to low main culmination (End of Act Two) to final victory, or (2) low midpoint to a high main culmination to tragic end. This simple story structure creates a solid foundation for plotting the second and third acts by framing the major peaks and valleys of the script. Later, when filling out everything in between, it’s important to provide minor victories and failures along the way.
But since most films take a protagonist who accomplishes the ultimate objective, it is easier to study and familiarize yourself with the more common “happily ever after” structure; however, for those of you writing tragedies, the best thing to do is to read and watch lots of films with those unhappy endings: Cool Hand Luke, Chinatown, American History X, etc. Also, study the classics. Shakespeare uses the same device with his tragedies: Hamlet, MacBeth, Romeo & Juliet, etc.
The trick is to respond to that high point at the main culmination immediately - often using a twist or revealing a new piece of information - to push the protagonist into the new Third Act tension and begin his or her tragic fall. And the very best films do this all in the same scene using a classic REVERSAL technique.
Now to get you started, let’s take a look at the ‘midpoint mirror’ in action by analyzing David Fincher’s 1995 crime thriller tragedy Se7en:
Midpoint (Middle of Act Two)
Using a library list obtained from an FBI contact, Somerset (Morgan Freeman) and Mills (Brad Pitt) are led a man named John Doe, but when they visit his apartment, Doe (his face hidden) fires his gun at them. After a long chase, Doe hits Mills with a tire iron, keeping him subdued at gunpoint, but lets him live and flees.
(Low Point: Clearly, this is a low point for the two detectives. They alert the serial killer of their presence, allow him to escape, and Mills is beaten up in the process, the killer maintaining control by deciding to let Mills live.)
Main Culmination (End of Act Two)
Surprisingly, John Doe (Kevin Spacey) arrives at the Fourteenth Precinct covered in blood and turns himself in, confessing to five murders: SLOTH, GREED, GLUTTONY, LUST and PRIDE. However, Doe’s lawyer explains that if Doe can take Somerset and Mills to two more bodies (ENVY and WRATH), he will confess to all seven murders. The detectives agree.
(High Point: Both Somerset and Mills have literally won; they have captured the killer and he has confessed. However, due to their own desire to solve the rest of the killer's murders, they agree to play Doe's game, which leads them both to a gruesome and tragic end.)
Final Resolution (End of Film)
As the three arrive to the desert outskirts of the city, a van appears and Somerset stops it. The driver claims he was paid $500 to deliver a box. Somerset cautiously opens the box, but recoils in horror from what he sees. Meanwhile, Doe explains that he was guilty of ENVY and tried to “play husband” with Mills’ wife Tracy, but he took a souvenir instead: “Her pretty head.” Now it’s up to Mills to complete Doe’s masterpiece; shooting Doe in vengeance, Mills embodies WRATH.
(Low Point: Doe wins in the end, leaving Mills and Sommerset both emotionally broken and scarred forever.)