Expressionism is a modernist movement in theatre that originated in early 20th century Germany, characterized by distorting reality to evoke emotions. It rebelled against realism and significantly influenced later theatrical movements, focusing on themes such as spiritual awakening and societal failures. Key features include dreamlike aesthetics, abstract stagecraft, and unique characters and dialogue styles, with notable works including 'The Cabinet of Dr. Caligari' and 'Metropolis'.