In bird box, a sentient, alien entity manages to decode the frequency of our neurons. Our eyes capture vast amounts of continuous data, and the entity managed to unlock a rosetta stone that allows it to speak to our senses by manipulating our neurons to replicate functions that interpret specific ideas. All five senses (I believe the sixth sense is a myth) are reactive to the inputted data. The entity is malevolent, and it preys on our hopes and fears to manipulate us into ending our lives and risking the lives of others. Sometimes the entity forces suicidal ideas onto other people, and sometimes (this is what most people have missed) it pushes onto our senses the perception that someone is there when they're really not.
Specifically; Tom never contacted anyone over the radio, Tom and Malorie were never visited by a gang of cultists, Malorie was never attacked by a man while attempting to sail in a small boat to an assumed safe haven, the little girl died after the boat capsized, and Malorie's vision of the school for the blind was what she saw after dying from the fall down the hill.
These moments are depicted in the movie with clues given that they're not real.
The radio cuts at random as the robotic voice of Rick explains that to reach the safe haven they'll need to look to be able to circumvent the dangerous rapids. This was the entity communicating through the radio, apparently able to broadcast a voice, which provided unnatural assurances for a family that's essentially stranded in the middle of the woods. The voice emphasizes that Malorie will need to follow the sound of the birds, granting her the false hope of assuming that a given circumstance will ensure safety. It's awfully telling that Malorie was never able to contact Rick after that initial contact, and Rick couldn't offer any other indicators that his mind was of this world (an opinion of a random topic for example).
When the gang attacked the home of Tom and Malorie, it was the entity causing Tom and Malorie to hear and thereby sense their presence. Nobody was there however. During the firefight, Tom is apparently shot in the shoulder by one of the assailants. Tom however has no bullet wound in subsequent scenes, a fact curiously ignored by other reviewers. The fight that actually happened was that Tom killed what he perceived to be a threat, and in doing so granted himself false hope which left him vulnerable to being forced to commit suicide.
When the man attempts to make Malorie and the children remove their blindfolds, Malorie is clearly seen chopping at nothing with her machete in some scenes. She interprets that she did cut him deeply, and that he died. The entity cannot push a perception on us that doesn't fit with our sense of reality. If our altered reality interprets a dead body, the entity cannot revive that idea because it cannot reverse the closure that the death has caused. This may seem like a weakness, but it's apparently how the entity manages to permit our minds to become at ease and thus accept our own demise.
The little girl was given a bell to use to alert Tom and Malorie of where she is when they become separated. When the boat hits a submerged semi-truck and capsizes, the bell is seen lying by the riverside as we can still hear it ringing. Reviewers seem to have interpreted this moment as if the girl simply came to the riverside and dropped the bell. However; the bell can be heard ringing as it's obviously not in anyone's posession. This was Malorie and the boy being manipulated to hear the bell ringing.
It's depicted that Malorie eventually manages to locate both children and they follow the sound of the birds until eventually reaching the school for the blind. In the book, Malorie and the children find a home where all occupants took the precaution of gouging out their own eyes, which is seen as a darker ending. I feel that the movie has a much darker ending however.
There is no school, there was no safe haven. While frantically trying to locate the little girl, Malorie falls down a hill and seems to be momentarily unresponsive. I feel that this was where she finally let go, and adopted the altered reality that the entity can create. The movie depicts the school as a warm, bright, and beautiful place where they can finally be free. It's indicated throughout the movie that the entity intends to communicate that they'll see something beautiful if they uncover their eyes, specifically when Malorie is attacked by the non-existent stranger while in the boat. Malorie is also reunited with a woman she had not seen in five years, her OBGYN Dr. Lapham. Seeing her gutted me, because at that point I was still holding out hope for a happy ending.
To satisfy certain plot holes, the entity can't show you someone who died, but it can project their voice. Early in the movie, a woman enters a burning car after saying "Mom?" and it's later explained that her mother died ten years prior. I mention this because I feel that the entity is limited in its capability to present imagined individuals to someone.
Also; Gary's actions proved that the entity is especially malevolent in that it can push a living being to act on its commands when a certain level of symbiotic understanding is met, as its emphasized that only after drawing images of the entity that he was motivated to act on behalf of the entity by forcing the other occupants to see the entity. One might assume that Gary wasn't real, except that he (unlike the examples I gave) is capable of manipulating objects like the window coverings and the can that's used to strike Tom.
The director had explained that Dr. Lapham survived because she (the director) wanted to give a very happy ending to viewers and showing that someone could survive against all odds would satisfy that desire.
I believe that Malorie wasn't fully at peace until she could be reunited with someone from her past, and she had already witnessed everyone else dying. I believe that the little boy, now falsely believing to be in the presence of the little girl, eventually removed his blindfold and succumbed to the entity.
I haven't seen a single review that entertains the idea that in such a profound doom scenario, everyone can die. That's not how stories are supposed to work, not for our security anyway. Specifically though, as a new father, the scenes involving the children left me as a mess mentally until I could recover.
They finally made a movie that broke me. I'll be fine, but I never thought that a movie could paralyze me like that.