Why is Macbeth so interested in Banquo's plans in Shakespeare's Macbeth?
As part of his growing paranoia, Macbeth is concerned about Banquo's plans because Banquo has also heard the prophesy about Macbeth and may suspect him of regicide. Also, the witches told Banquo that his sons will be kings, and Macbeth wants to prevent this prophesy from being fulfilled by having him followed and killed.
In Act III, Scene 1, Banquo arrives at the palace and speaks to himself about Macbeth. He remarks that the prophesy of the three "weird women" has come true for Macbeth: He is now king. But Banquo also recalls that the witches told him that he would sire many kings, not Macbeth. Just then, Banquo hears Macbeth approaching and he becomes silent.
When Macbeth enters, he greets Banquo and asks him if he is traveling this day. Banquo replies that he is, but he will return in time to dine with the new king. As soon as Banquo departs, Macbeth orders an attendant to bring certain men before him. As he waits, Macbeth speaks to himself, saying to be king means nothing if he is not safe as the king:
... Our fears in BanquoStick deep, and in his royalty of natureReigns that which would be feared. 'Tis much he dares,And to that dauntless temper of his mindHe hath a wisdom that doth guide his valorTo act in safety. There is none but heWhose being I do fear, and under himMy genius is rebuked.... (3.1.50-59)
Then, the murderers enter. Macbeth asks them if they have seen the proof that it is Banquo who has dealt them injustice. They reply that Macbeth has made it known to them and they are ready to take whatever action he wants because they are both so weary of blows and disasters that they are not concerned about what may happen to them. So, Macbeth solicits them to kill Banquo and his son Fleance.