The entire movie is a one-take. Even though there could have been cuts when the camera was put on the ground. Erik Poppe had five days to try to get the perfect take and could not attempt more than one take a day. The take from the fourth day has been used for the final movie.
The take from the third day could not be used because there was a solar storm that interfered with all the signals of the equipment. This made it impossible for the director to communicate with his cameraman.
The movie wasn't filmed on Utøya itself but on the neighbouring island.
The movie features an 83 minutes long take which depicts the events in real time.
The film is based on historical events, but the characters are fictional.