Irony

Duncan is all about the fact that he owes Macbeth more than he can pay,

Thou art so far before
That swiftest wing of recompense is slow
To overtake thee. Would thou hadst less deserved,
That the proportion both of thanks and payment
Might have been mine! Only I have left to say,
More is thy due than more than all can pay.

We all know that Macbeth intends to make Duncan pay him with his life. Duncan also says that he will make Macbeth an important man in the court. The audience knows the irony that Macbeth intends to be the most important man in Scotland: he intends to usurp Duncan.