Shakespeare characterizes the three Weird Sisters by allowing them to establish the mood of the play in the very first scene. Right away, the dark and ominous mood is set with thunder and lightning and these three creepy women making plans for what they are going to do after the battle's over; these plans sound somewhat nefarious. During their conversation, they also speak to their familiar animal spirits—"Graymalkin" and "Paddock"—a very strange and off-putting behavior.
In addition to interacting with these spirits, the sisters' speech pattern helps to establish their characterization as well. They speak in rhyming couplets, a pattern that sounds hypnotic and spell-like: "witchy" if you will. They also speak in a particular rhythm called trochaic tetrameter. This means that there are four (tetra-) feet per line and each foot is called a trochee (which is two syllables: one stressed syllable followed by one unstressed). Most of the other characters in the play speak in iambic pentameter, a meter made up of five feet per line (penta-) where each foot (called an iamb) has two syllables: one unstressed followed by one stressed‚—the opposite of a trochee. Because the Weird Sisters' speech begins on a stressed (or accented) syllable, unlike the vast majority of other characters, it sounds more menacing and aggressive, an auditory clue to their menacing and aggressive characters.
Finally, in the last couplet of the scene,
Fair is foul, and foul is fair;
Hover through the fog and filthy air,
the Witches employ alliteration, the repetition of an initial consonant sound. The wispy, misty, snaky "f" sound begins six words in these two lines alone. This sound is airy rather than vocal, adding to the sisters' mystery. All of these auditory clues—the rhyming, the meter, and the alliteration—seem to point to the fact that these women should not be trusted; even their use of paradox in the first line of the quotation above seems to confirm this. They speak in riddles, they seem to delight in mystery and menace; nonetheless, Macbeth will believe them. We learn from them, and from their paradox, that appearances are often deceiving.
The memorable Three Weird Sisters in Shakespeare's Macbeth perform a significant role throughout the play by manipulating Macbeth to make rash decisions which lead to his tragic downfall. In the opening scene of the play, Shakespeare introduces the audience to the Three Weird Sisters, who discuss meeting Macbeth when the battle is over before delivering their famous line "Fair is foul, and foul is fair" (Act I, Scene 1, line 12).
Thematically, this statement introduces the concept that nothing is what it seems, which is significant to the plot of the play. Continually, Macbeth and other characters will be fooled into believing that things are opposite of their true nature. In Act I, Scene 3, the Three Weird Sisters discuss how they will avenge a woman who refused to share her chestnuts. They mention that they will influence the winds to make her husband's journey across the sea difficult. Throughout this conversation, Shakespeare indirectly characterizes the witches as being petty and vengeful. The fact that they have the power to influence the weather, yet lack the ability to directly harm or kill the woman's husband suggests that their control over people's fates is limited and ambiguous.
Later in the scene, Macbeth and Banquo meet the Weird Sisters and are repulsed by their appearance. Banquo comments,
What are these so withered and so wild in their attire, that look not like th' inhabitants o' th' Earth, and yet are on ’t?—Live you? Or are you aught that man may question? You seem to understand me, by each at once her choppy finger laying upon her skinny lips. You should be women, and yet your beards forbid me to interpret that you are so (Act I, Scene 3, lines 39-48).
The witches proceed to prophesy about Macbeth's eventual kingship and Banquo's descendants before disappearing into thin air. During their encounter, Shakespeare characterizes the Three Weird Sisters as grotesque, evil characters who use their knowledge of the future to ruin Macbeth.
In Act IV, Scene 1, the Three Weird Sisters concoct a charm that will negatively affect Macbeth's ability to interpret their prophecy accurately by using various animal parts and crude ingredients as they chant, "Double, double toil and trouble, Fire burn and cauldron bubble" (Act IV, Scene 1, lines 20-21).
When Macbeth enters the scene and demands to know his future, the witches conjure three apparitions that give Macbeth a false sense of confidence. The nature of the witches is nefarious, and their prophecies are ominous. Their function is to the set the tone of the play and confuse Macbeth into making terrible decisions that will result in his demise. Overall, Shakespeare characterizes the Three Weird Sisters as wicked, vengeful, grotesque witches whose sole purpose is to cause chaos and trouble.