The use of metaphor in drama is a complex device used by playwrights to draw a comparison between two seemingly dissimilar things. A famous metaphor occurs in Arthur Miller’s play The Crucible, where the play’s subject matter about the Salem witch trials of 1692/3 is actually a metaphor for the world in which the playwright was living at the time, the witch hunt for Communists in 1950s America (McCarthyism). This parallel, where the qualities of the first event (Salem witch hunts) are attributed to the second (Communist witch hunts), is a comparison, which in turn forms an image in the minds of the audience watching The Crucible.
Dramatic metaphor should not be confused with symbol, although the two are similar. Symbol often, but not always, involves an object that is substituted for something else. For example, a rose (object) on stage is substituted for love (feeling). The literal meaning of the object is that it is a flower, but the symbolic meaning of this object is far greater and depends closely on the characters, plot, dialogue, setting and context of the drama. There is no comparison between the rose and love, but rather one is replaced for the other. In dramatic metaphor, however, a play title, event, line of dialogue, image or setting is compared to something else in order to enhance its meaning. In plain terms, the use of dramatic metaphor could perhaps be understood as an extension or more complex form of symbol.
Below is a short video from England’s National Theatre involving several UK playwright’s discussing the use of metaphor in drama (including The Crucible example) which should prove useful for teachers and students of drama.