What impact does the use of trochaic tetrameter by the witches in Macbeth have on their depiction?

Well there are three forms of speaking in Macbeth. Iambic Pentameter (the way the nobles speak), trochaic tetrameter (how the witches speak) and prose how everyone else speaks. Prose obviously enough sounds fairly normal, after all we generally speak in prose. Iambic Pentameter sounds similar but the clear stress pattern lends it a more formal, educated, tone – it reflects the characters. Trochaic Tetrameter however is a very un-natural way to speak. It lends their chant (already not exactly nice considering it contains "eye of newt" and "tongue of dog") a sing-song and rather uncomfortable air. This marks them out as being something outside normal life, they could be chanting about the washing up and it would still be nerve jangling. So it marks them out as strange, dangerous and otherworldly all by just the stress pattern in which they speak.