Expressionism is not really a program or defined movement with a particular group of writiers. It is more of a reaction to the realism and naturalism writing and philosophy.
Expressionist painters and writers concerned themselves with feelings and the inner workings of the emotions and how this is expressed in art. Vincent Van Gogh is considered an expressionist painter. Franz Kafka's novels, Frank Wedekind's plays such as Pandora's Box upon which Alban Berg's opera, Lulu was based.
Naturalism tends to pit humanity in a fated match against nature with the natural world being harsh and inhospitable. Humans are viewed as helpless or passive victims of the natural environment. Naturalism began in France with the writers Jules and Edmond Goncourt with their novel Germinie Lacerteux (1865), was later led by Émile Zola and his novels Thérèse Raquin (1867), Germinal (1885) which explored the human drive of hunger, sexual obsession and heredity defects.
Both of these literary movements were spawned by the German philosopher Nietzsche. He is probably most well known for the quote: "That which doesn't kill me makes me stronger."