In Macbeth, Shakespeare employs at least two expository techniques in I.i. The first is the paradoxical couplets which bracket the act. Both couplets suggest contradictory but nonetheless true situations. ll. 3 & 4 reference the obvious physical battle currently being fought but, more importantly, they indicate the battle for Macbeth's soul which is being waged by the witches without his knowledge ("When the battle's lost and won"). "Fair is foul, and foul is fair" states the obvious theme to be developed: namely, Macbeth, Duncan's beloved champion, will murder him in the very castle where he was warmly welcomed. And, eventually, all fairness will be blotted out by "the fog and filthy air."
The manipulation of time and place is another device used by Shakespeare. The first scenes tell us that the witches' "vetting" process of Macbeth has been going on for some time in the past. The witches appear briefly in the present and they designate the future meeting time and place--"set of sun," "upon the heath"--for their encounter with Macbeth. As rapidly as they appeared, they vanish to meet their talismanic cat and toad.