Short answers, pulled from the story.
Naturalism in theatre is a movement that developed in Europe in the late 19th and early 20th centuries. It aimed to create an illusion of reality through dramatic and theatrical strategies, grounded in the study of human psychology, heredity, and environment. The movement was first explicitly advocated by Emile Zola in his 1880 essay Naturalism on the Stage.
August Strindberg's Miss Julie, written in 1888, is considered the most successful example of naturalist theatre. Strindberg wrote it to follow both his own version of naturalism and the principles described by Emile Zola.
Zola described three principles, expressed in French as faire vrai, faire grand, and faire simple. First, the play must be realistic, built from careful study of human behaviour and psychology, with characters shaped by heredity and environment. Second, the conflicts must be of life-altering significance, not petty. Third, the play must be simple, free of complicated sub-plots or lengthy expositions.
Naturalistic writers drew on Darwin's theory of evolution to argue that heredity and social environment determine a person's character. Darwinism pervades naturalistic plays in the determining role that environment plays on character and as motivation for behaviour.
Key naturalist plays include Georg Buchner's Woyzeck (1837), Leo Tolstoy's The Power of Darkness (1886), August Strindberg's Miss Julie (1888), Gerhart Hauptmann's The Weavers (1892), Anton Chekhov's Uncle Vanya (1898) and The Cherry Orchard (1904), and Henrik Ibsen's A Doll's House (1879).
Realism seeks to describe subjects as they really are, while naturalism goes further by attempting to determine scientifically the underlying forces, such as environment and heredity, that influence human behaviour. Naturalism also explicitly opposed romanticism and excluded supernatural, historical, or mythic elements from the stage.