Watch it again and look for the staging.
The first time you watch it, you are swept in by quick editing and emotional screaming. Your mind tells you that what they are experiencing is real. But watch it without the sound and you can see the cheap acting and the staging.
There are too many giveaways showing that these 3 are actors.
My favorite is the part when they are screaming inside the coffin for their very lives, but can't knock the casket over on the ground from where it is perched? Nor pull off the metallic blankets? They are paralyzed and can't move?
If anybody was getting locked in a casket, their first instinct would be to use your hands and get out of the blanket. They don't. Then, when inside, try to push your way out. None of the actors do that. They just lay there the dark screaming, but not using their hands. It's all staged.
