DAVINA: When Duncan says that it's impossible to judge what someone is thinking just based on their outward appearance, he's referring to the traitorous Thane of Cawdor, who Duncan had trusted completely.

RALPH: Ironically, at that precise moment, Macbeth walks in and receives the king's praises – the same Macbeth who has been envisioning Duncan’s murder.  So it may turn out that Macbeth is not the person the king thinks he is, either.

DAVINA: Members of King James's court would have detected some additional irony in this passage. James prided himself on his ability to use stratagems and role-playing to test the loyalty of those around him.

RALPH: I can just imagine King James saying to himself at this point in the play, "Well, Duncan, maybe you don't have the art to uncover a mind's construction in the face, but I certainly do!"