Perhaps existentialism is a philosophical movement that doesn't particularly seek to establish any particular 'truths' and, in a way, rejects the notion that 'truth' matters. While expressionism is primarily an arts movement that privileges subjective experiences, and infers that they can provide some sort of 'truth'.

Expressionism may be part of the paradigm which is existentialism. If ideas can move people, then expressionism may be the result of how existentialism moved the expressionist to paint. The angst, imagination, and focus on the subjective are all related to existentialism, however may differ in small ways. Existentialism focuses more on the subject, but also how the subject comes to terms with the objective world. Expressionism makes one feel as though they still retain some humanity. They aren't acting as machines working in an assembly line, where they act as mere meat bags, thereby producing food for consumption. To express is something that comes from within, it's intrinsic by nature. So expression is in contrast to mechanical ornamentation (doing the same thing over and over again). Existentialism embraces the former. However, existentialism is far broader in thought and diversity. Existentialism wrestles with questions of knowledge, conduct, and governance as does any well developed philosophy that thrives to become "complete".

Expressionism was primarily an art movement,most visible in the form of paintings and cinema.Also its philosophy was in concomitance with modernist times.While existentialism has been most profoundly been reflected in literary works...its philosophical underpinnings are also more in tandem with post modernist philosophy...

An expressionist work would typically be a radically distortionary representation of reality in a work of art based on the difference of perception. But an existentialist text or work would enjoin upon the viewer or reader to revisit the very idea of existence