The function of expressionism is to represent abstract ideas
through concrete forms
For example, in Eugene O'Neill's play, Emperor Jones, Jones is haunted by his fears, in particular feelings of subordination. O'Neill represents this by putting Jones on the block at a slave auction. The auction isn't "really happening," there in the middle of a jungle, but the script makes no indication that the auction is fantasy. In contrast, a realistic play would indicate the scene as a dream, a flashback, or some other dramatic convention with a defined break "in" and "out" of the play's action.
O'Neill was the first American playwright to use expressionism, and he claims to have been uninfluenced by the concurrent work in Germany in the 1920s.
A popular (though pretty much unconscious) misconception about expressionism is that it depends upon wacky colors, shapes, language, ideas, forms, etc to serve its function. Rather, an expressionistic work is freed from the conventions that force realism into nonwackyness. That's just my two cents.