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.
Stage directions in this play reflect the dramatic techniques O'Neil uses in every scene. As an Expressionist play, it is structured in an erisodic form that follows the hero's vision and experience.The stage directions of every scene establishes the detailed environment and setting of the play as it is seen by the central character(Yank). For example, we can see the harmony Yank feels with the steel and the metallic noise of the ship when he is convinced of his power at the beginning of the play. Then in the fifth scene, we are aware of the unrealistic treatment of the characters and environment in 5th avenue.
And, in giving detailed descriptions of setting combined with unrealististic techniques, O'Neil combines techniques from Realism and/or Naturalism ( which are characterized by a detailed, realistic treatment of setting) with Expressionist techniques (like using anonymous types, subjective vision, syntactic compressions, monologues, symbolic figures or picture sequences, and the episodic structure)
O'Neil also uses his stage directions as a way to avoid any confusion about the play: the treatment of the scenes should not be Naturalistic. This probably means productions of this play should not reproduce the "natural" surroundings in a normal, realistic way. Actors should not strive imitate human behavior in the realistc way. In some Expressionist plays, there are lopsided stage sets and shadowy lighting to enhance this anti-naturalist attitude.