This whole film is shot and made to feel like a single video tape that was found after the event and played back for you without editing.
The idea is that a group of friends are having a "going away" party for one character. A friend, Hud, gets handed a video camera to record the party and personal messages from the party guests for the dude who is leaving. Then the shit hits the fan. The guy with the camera just keeps rolling. And that is the film.
At no point do you see anything shot from any other perspective than that one video camera. And that camera work is designed to feel like a home video level of operator skill. It is shaky. There is running with the camera. Zooming in and out. Extreme close ups. And a number of other things that average people do with camcorders that we don't typically see in professional movies.
This is a visual roller coaster and the ride is almost as chaotic and terrifying for the audience as it was for the characters. Which brings me to characters and acting. The acting is not typical in that nothing felt scripted or rehearsed. People say stupid, obvious, repetitive nonsense as we might expect real people to do in horrible circumstances. They aren't action heroes. They are very scared and normal folks trying to stay alive. Although you do feel for the characters, I think that has more to do with being drawn into their experience than it does with any of the characters being all that interesting or compelling. Like the camera work I think this was intentional.
I don't want to spoil any of the surprises, so I'm not going to comment much further. I will say that within the limitations of the camera work, the effects are awesome. At no point did I think, "Oh, that looks so fake." Nope, everything looks like video shot by some citizen who was actually there. Which was the point of the movie (I think) and if so then the filmmakers did a great job.
Last note. Watch through the credits. Once the music stops and the text has scrolled away, listen closely.