Anders Ohlin (Pelle 'Dead' Ohlin)'s brother was very supportive of the movie and one of the pairs of jeans that Jack Kilmer's character wears is a pair that belonged to Pelle. All of the other clothes were designed from scratch but those particular jeans were legitimate.
Euronymous wasn't a big drinker and, according to all sources, drank a vast quantity of Coca Cola (which is visible throughout the film).
From 1983 to 1984, director Jonas Åkerlund was a rehearsal drummer for the Swedish metal band Bathory, which was one of the inspirations for the Norwegian Black Metal bands of the 1990's which are t he focus of this movie. He stated in a 2018 interview that when Black Metal culture developed in the 90's and become "too serious", he left the music scene and moved on to film-making.
Euronymous is seen smoking cigarettes throughout the film. However, he was a non smoker.
Director Jonas Åkerlund has stated in a 2018 interview that he used little black metal music in the movie in part because "it's kind of painful to listen to black metal music if you're not used to it or don't love it."