Eugene O'Neill was an American playwright who wrote during the American Realist movement. Realists believed that a person was simply a person, within realistic environments/happenings, and dialect which mirrors the period/locality of the text/play. Realism did become very important in theatre during the period focusing upon "altering practices of set design, costuming, acting style, and dialogue (eNotes--Realism Study Guide).

O'Neill's use of stage directions was very important given his attention to detail. In the stage directions for act one of "The Hairy Ape," O'Neill is very specific about the removal of any Naturalistic ambiguities which could be made. Instead, he is very specific about depicting life and the characters as they would really be (as Realists desired).

The treatment of this scene, or of any other scene in the play, should by no means be naturalistic. The effect sought after is a cramped space in the bowels of a ship, imprisoned by white steel. The lines of bunks, the uprights supporting them, cross each other like the steel framework of a cage. The ceiling crushes down upon the men’s heads. They cannot stand upright. This accentuates the natural stooping posture which shovelling coal and the resultant over-development of back and shoulder muscles have given them.

Here, O'Neill wants to insure that nothing imaginary, colored, or figurative is allowed on the stage. Instead, through his very detailed directions, no mistake can be made regarding what O'Neill wishes the viewer of the play to see.