Schoenberg was part of the 2nd Viennese school of composers, and originally composed in a romantic style. He then abandoned this late romantic style of composition in favour of music that used extreme chromaticism and dissonance. Der Kranke Mond is an expressionist piece, much like distorted images in art (Munchs ‘The Scream’), and use of violence in drama.
There are only 2 voices used in this piece- flute and voice. They are both independent lines of music, yet both equally important. Ranges are used to extremes in both parts.
There are no recognisable motifs, and the pulse is undetectable. This makes the piece feel unsettling. Rhythm is not a key feature!