The most compelling drama of Act One of Shakespeare's Macbeth is the allure of the witches in drawing the once honorable and highly respected Macbeth into a plot to kill Duncan—his King, his cousin and his friend.
In Act One, scene one, the witches set the mood for the play. In Shakespeare's time, the Elizabethan audiences very much believed in the supernatural—in witches, fairies and ghosts. Along with a belief in these beings, people also believed that the powers of evil worked hard to win honest souls to their eternal damnation. (This belief was carried over into American literature—the influences seen in works such as Young Goodman Brown by Nathaniel Hawthorne.) The presence of the witches would most certainly have made a strong impact on the viewer—just as a monster or murderer would in the opening scene of a movie and television show with today's audiences. It is noted that in Macbeth...
...ambition conspires with unholy forces to commit evil deeds...
In scene one, the witches announce that they will meet again, seeking out Macbeth. This element is essential to creating this compelling drama. As we learn more about Macbeth and his vaulting ambition (his need to be great), we realize that the play would never have worked had the witches met Banquo, who was also a loyal general for Duncan, but was stronger ethically and morally.
The witches also use language that sets the tone, referring to...
...the fog and filthy air. (12)
Besides the fact that a war is being waged, evil is in the air: it's not just foggy, it's "filthy," a word that is easily associated with the witches.
In scene two, Duncan is given a report of the valiant way Macbeth defeated the traitorous Macdonwald and widely praises him. There is no question of the depth of Duncan's respect for Macbeth, allowing that no one will suspect the monster Macbeth is to become.
In scene three, the witches prove themselves to be as diabolical as one might expect, describing the poor sailor they have tormented, and casting a spell.
Then Macbeth and Banquo appear as expected. Here the witches make their predictions, which (in themselves) are not dangerous; but Macbeth's immediate preoccupation is when the title "Thane of Cawdor" is given to him. Macbeth imagines that if the witches told the truth of Cawdor, he will also be king—even though it would mean Duncan would have to be dead.
Meeting the King in scene four, Duncan praises Macbeth, but also names his son Malcolm as his heir. Macbeth notes that this is an obstacle in his path, showing that he is already planning to make his way to the throne, one way or another. By scene five, we learn that Lady Macbeth will do anything to make sure her husband becomes King. She is also set on murder as she describes...
...the fatal entrance of Duncan (40)
Lady Macbeth also calls on evil forces:
Come, you spirits
That tend on mortal thoughts…
And fill me, from the crown to the toe, top-full
Of direst cruelty! (41-44)
By scene six, Duncan is staying at Macbeth's castle. Macbeth is unsure about the assassination, but Lady Macbeth is not. She swears that she has the mettle to kill her own child if it were necessary. Macbeth agrees to proceed: his path is set to commit regicide.
I am settled, and bend up
Each corporal agent to this terrible feat. (90-91)
The compelling drama is the evil and death introduced by the witches—coupled with Macbeth's ambition—that will galvanize the play toward its tragic conclusion.