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The opening is extremely atmospheric, with witches coming out and whispering of their allegiance to Hecate. When a story starts this way, then the first thing the audience is led to expect is that something bad/evil is at work. And then they make mention of meeting Macbeth soon and of bad things to come. It puts in the audience's mind the expectation that something evil will happen, and so the audience will be looking for that in every subsequent scene - where is the evil coming?