How does the author use foreshadowing in The Man Who Turned Into a Stick?

As soon as this play begins, the offstage voice points toward one of the major themes: that things are not always as they appear. Speaking of the ability of Mona Lisa's smile to conceal the realities of both the past and the present, the voice foreshadows the fact that a tension between what appears to be true and what is true will underlie the play's action.