Expressionism deals with a lot of ideas, mostly dark and leading to the emotions like anger, sadness, regret, or betrayal. Concepts like death, war, and love all seem to be constant topics for Expressionist work.
Expressionism is completely emotionally driven, which is evident in its lack of traditional structure. Emotions are not neatly organized and understood, and Expressionist art is the same way. There is no formula for creating and understanding emotion, and therefore there is no formula for creating and understanding Expressionist art.
The outcome of Expressionist work guarantees a rush of emotion for the viewer, and only suggests that the viewer thinks. Emotion comes first, and thoughtfulness is merely an afterthought.