Subliminal editing

In Fight Club, following a life that leaves him deeply unsatisfied, the protagonist creates an alter ego — played by Brad Pitt — who embodies everything he himself cannot be, a sort of super-ego.
The example shown above depicts the first true introduction of this character, who will later be presented to the audience as he appears to the protagonist: a real, tangible person. Only at the film’s conclusion will both the protagonist and the audience discover the character’s true nature.

Fincher inserts a ghost frame of the alter ego during a scene in which the protagonist is speaking with a doctor about his insomnia.
Sleep deprivation is a frequent cause of hallucinations, and within the film’s context, this brief flash frame serves two purposes:

1. It symbolizes the birth of the alter ego within the protagonist’s mind (reinforced by the fact that the shot is from his subjective point of view).

2. It acts as a clue for the audience — a subliminal hint that, when Brad Pitt’s character reappears later, he will behave as though he were a real person, interacting with others throughout the story and misleading both protagonist and viewer alike.

This insertion effectively plants a seed in the viewer’s mind.
It’s worth noting that the same film later contains a scene that openly references this idea: when the character splices a pornographic frame into a family film. Though played for humor, the moment functions as a meta-commentary — a direct explanation of how subliminal frames work and how they shape perception.
This “joke” thus becomes a key to understanding the film’s narrative logic.

On a subconscious level, the viewer has glimpsed a crucial clue — even if only for a fraction of a second — through which one of the main characters has been introduced.
While the rest of the story will work to mislead us into believing he is a flesh-and-blood person (just as the protagonist does), the final revelation feels both logical and intuitively satisfying.

A different kind of subliminal editing appears in Se7en (1995).
One of the film’s unique qualities lies in how it depicts the murders committed by the serial killer.
The first killings are shown explicitly and in graphic detail, while the final ones are concealed from the viewer, culminating in the infamous climactic moment: the delivery of a box containing the severed head of the detective’s wife — which we never actually see.

The horror, however, is powerfully felt. Why? For two reasons:

1. The explicit violence of the earlier murders allows the audience to vividly imagine the unseen horror of the final act, guided by the detectives’ descriptions.

2. The film carefully builds empathy for the detective’s wife, particularly in a quiet, intimate scene where she confides her doubts and fears about her pregnancy to her husband’s partner.
   This moment of vulnerability, previously unknown to us, directs the viewer’s emotions toward a predetermined destination — grief and shock at her death.

In the final confrontation, the detective faces his wife’s killer and, in that instant, learns that she was pregnant — a fact he himself did not know.
Torn between duty and vengeance, the editing inserts three fleeting frames of his wife’s face at the precise moment he looks away.