Expressionism emerges during waves of violence, 1
From the turn of the 20th C. through the early 1930s, there was a crime wave that went against the centuries-long decline in violence. It was somewhat uneven, hitting some countries and not others, or some countries only for a portion of the period. (Here in America, the homicide rate rose steadily from at least 1900 to a peak in 1933.)
Times of rising violence rates produce visual art that is, for lack of a better catch-all word, more Dionysian. Specifically, it is more emotionally open, looks more dynamic, shows a greater variety and intensity of colors, and chooses subject matter that is more dramatic, beautiful, and sublime.
So, as violence rates cycle up and down, the dominant art styles will follow. I covered this before in the case of Art Deco and Fauvism both being born in the early 20th C. crime wave and seeing revivals during the more recent crime wave of ca. 1960 to 1990, in contrast to their polar opposites reigning during the falling-crime period of the mid-century (such as dumb ugly box architecture).
Expressionism shared most of the core traits of the art of its time, such as Deco and Fauvism. Its variation on them was to dial up the raw emotion (often with exaggerated poses), to heighten to subjective point-of-view by using unusual or distorted perspectives, and to focus more closely on the theme of social alienation and disintegration.
Typical of art from apocalyptic times, it doesn't convey a sense of nihilism or fatalism. Quite to the contrary, it shows a deep yearning to actively reach out and re-connect with our fellow group members and with nature. There's an anxiety about this, though -- having slid so far toward atomization, we may have to embarrassingly grope our way through this re-connection at first. Still, what else are we supposed to do when the end of the world looks so near? There's no hint of the whiny-emo complaint that "No one wants to be my friend, please play with me," but rather "Our society keeps getting more fucked up, so we'd better stick together and try to do something about it, or else we're goners."
Here are two representative paintings from Ernst Ludwig Kirchner and Franz Marc, and two stills each from The Cabinet of Dr. Caligari and Metropolis:
Before getting to the post-1960 period in part 2, let's close with a reminder of what direction the art world went in during the mid-'30s through the late '50s, when violence rates began plummeting once more. Painting and sculpture gradually became stripped of emotion, felt static, used fewer and more muted colors, and "portrayed" subject matter that was so abstract that it could not strike a dramatic chord in the viewer. Although this trend declined during the '60s with the rise of Pop Art and psychedelic art, it was still common enough in the form of Minimal and Conceptual Art. In all these respects it was not very different from the soulless mid-century architecture.
Here are some representative works by Jackson Pollock, Mark Rothko, and Donald Judd.