The part of the play the witches take part in is indeed a role difficult to define. They do not control fate, nor are they mere foreseers of the future. They have the role to develop matters further whether it would be to set the tone of the play or to help unleash Macbeth's most secret desires.
The witches are the first characters the audience is first introduced to in the play. .
The audience is immediately given the impression that these witches are unlike humans, they all speak in rhyme, rhythm and repetition, crying "When shall we meet again?/ In thunder, lightning, or in rain?" .
Their preferred place of meeting are during unpleasant conditions, a lot of what they speak is in the future tense, of events that have yet to occur. All witches speaking in unison, and with the alliteration with the repeated "f" sound, they give prominence to their observations, "Fair is foul and foul is fair. Hover through the fog and filthy air." Establishing the main theme of the play, that things are not what they seem, that good is confused with evil, and neither can be distinguished from the other. .
Their speech is riddled with equivocation, causing Macbeth to misinterpret their meaning. Banquo who sees through the witches, says, "The instruments of darkness tell us truths, win us with honest trifles, to betray's in deepest consequence.".
The witches are the physical forms of evil, full of wickedness and mischief. They have otherworldly powers and behaviour that humans do not possess, yet here they are, dwelling among them, meddling in their affairs. As questioned by Banquo, "They look not like the inhabitants o" the earth/And yet are on't?" .
Their evil is revealed as they discuss with glee the destruction and disorder they have created on land and sea. "The Weird Sisters, hand in hand/ Posters of the sea and land/Thus do go, about, about/ Thrice to thine, and thrice to mine, and thrice again to make up nine.