A thesis statement for a play, much like a thesis statement for an essay or research paper, should attempt to quickly and efficiently summarize the central concept of the play. For example, a thesis statement for William Shakespeare's "Hamlet" might read: "A young prince, returning home after his father's death, seeks revenge on his uncle, a murderer and usurper of the throne." Each scene in the play either shows Hamlet's progress or the forces that prevent him from achieving his goal.
Plays often need to be rewritten or edited during early productions, as problems with scenes become evident once the play is staged by actors. Having a thesis statement written before you attempt a rewrite can prove valuable, since rewrites can focus on scenes that directly relate to that thesis statement. If a scene doesn't seem to support the thesis statement, you might consider cutting it so that it does not detract from the rest of the work.
A thesis statement can also benefit the actors in your play, especially if the play has gone into wider distribution and the writer is not present to answer questions. Having a thesis statement can help identify character motivation or the purpose behind certain scenes. If the thesis statement states that jealousy is central to the action of the play, for example, an actor can keep that concept in mind while playing a character.
A thesis statement can also be used when creating publicity for a play. When a writer, director or actors are promoting a new play, a thesis statement allows them to succinctly and powerfully share what the play is about. You can use a thesis statement to begin a publicity campaign, even using powerful phrases of the statement in advertisements. For example, an advertisement for Hamlet could call it "a story of revenge and murder," concepts drawn directly from the thesis statement.