How does the language and choice of verse in Macbeth shape reader response?
Shakespeare's choice of verse and meter for the Weird Sisters certainly shapes our response to them right away. In Act 1, Scene 1, we meet them, and they are speaking in verse, with end rhyme. The rhyme makes their words sound very chant-like, almost as though they are casting a spell (which is appropriate!). Further, they speak in trochaic tetrameter, a very aggressive-sounding meter compared to the unrhymed iambic pentameter (or blank verse) that Macbeth and the other nobles use to speak. Tetrameter means that there are four "feet" per line of speech, and trochaic means that each foot has two syllables, an accented syllable followed by an unaccented syllable. In other words, most of their lines begin on an accented syllable and this adds to our feeling that they are quite menacing and even malicious. When other characters speak, their lines begin with unaccented syllables making them seem a great deal less strange and ominous than the sisters.
Moreover, the fact that the sisters speak in paradoxes and with alliteration of the "f" sound also helps us to understand that they are mystical and should be interpreted by us as evil. To say that "Fair is foul, and foul is fair" really doesn't make much sense: how can something be both fair and foul at the same time? Once we understand that fair or good things may seem bad, and bad or foul things may seem good, we can understand the paradox; however, we really don't gain this understanding -- that appearances can be so deceiving -- until sometime later. For the time being, then, these sisters are very mysterious and dark, and we feel as though we do not understand what motivates them or their menace. Likewise, the repetition of the "f" sound in the words fair, foul, fog, and filthy (in lines 12-13) sounds similar to a hissing snake and also sounds sort of dirty and base. This choice helps to impress us with their foulness as well.
Interestingly, after Macbeth has finally (re)committed to the plan to kill Duncan, his speech mimics this repetition, as if to show that the Weird Sisters' manipulation of him is complete and he has turned to the dark side. He says, "mock the time with fairest show. / False face must hide what the false heart doth know" (1.7.81-82). The fact that he now speaks with alliteration of the "f" sound helps us to understand that he possesses a great capacity for darkness as well.