Theme refers to a play's central idea, that statement about life or human nature which is conveyed through the plot, characters and setting.
The title of a play (A Doll's House, for example) may give a clue to its theme.
Recurring images, actions and statements usually point to a unifying theme. For example, in A Man For All Seasons the imagery of tides and flowing water and those who float with them contrasts with the land-based solidity of Thomas More who finds the law both a forest thicket and a causeway. The theme of adamantine selfhood versus the common life of equivocation is thus nicely elaborated. In Waiting for Godot the recurring references to physical hardship (from sore feet and weak bladders to the crucifixion) emphasize the painful isolation of existence. The repetition of "waiting for Godot" forces us to ask who Godot might be and what this "waiting" might signify. These repeated images and passages are likely more easily identified in reading a play than in seeing it.