Duncan's reply emphasizes the theme of appearance and reality. He states,
There's no art
To find the mind's construction in the face:
He was a gentleman on whom I built
An absolute trust.
In essence, he is saying that it is impossible to determine a person's true character or motives by observing his expressions. There is no skill which can enable one to read another's thoughts. He maintains that it is on this basis that he deemed Cawdor a gentleman who he firmly believed in. Cawdor betrayed his king by supporting the rebel Macdonwald and the Norwegian Sweno by taking up arms in an attempt to usurp the Scottish throne. Since the Scottish troops had been victorious, the thane was to be executed for his treason.
It is also tragically ironic that the king had, in Scene 2, instructed Ross to greet Macbeth with the thane's title. Duncan had absolute faith in Macbeth, who supposedly showed his loyalty on the battlefields by fighting with great vigor -- slaying the traitorous Macdonwald by unseaming him from "the nave to the chops" and continuing the battle, even though the odds were against them, and achieving a hard-fought victory.
The irony lies in the fact that Macbeth, just like the preceding Thane of Cawdor, is a traitor. He purposely sets out to assassinate the king. He forced the king's sons, Malcolm and Donalbain, true heirs to the throne, into self-imposed banishment and then usurped the throne.
Macbeth then betrayed his country by imposing tyrannical rule on its citizens, spreading unexpurgated mayhem, bloodthirsty savagery, and disruption throughout the land. In the process, he almost destroyed Scotland and paid for his pernicious indiscretion with his life.