Aspect ratio
I’m surprised nobody is talking about this.
Not only did the director choose to shoot the movie in black and white, but he also abandoned letterbox for what appeared to be a pure 1:1 square frame. Basically, in terms of color & aspect the movie harkened back to the early days of cinema when the screen was square.
Personally, I think this was a bold move - The high contrast black & white gave it an otherworldy look and the square framing definitely added an element of claustrophobia that heightened the overall feeling of two guys shut in a small space and going berserk.