In King Duncan's speech in Act 1, why you think Shakespeare had Duncan say this just before Macbeth enters. Do you agree with King Duncan that you cannot tell what people are really like just by looking at them?

What does the king say? You just typed "this".

There's no art

To find the mind's construction in the face.

He was a gentleman on whom I built

An absolute trust.

*Macbeth, Banquo, Ross, and Angus enters*

O worthiest cousin!

The sin of my ingratitude even now

Was heavy on me. 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

Night have been mine! Only I have left to say,

More is thy due than more than all can pay.

They just finished executing the Thane of Cawdor for treason. King Duncan had totally trusted the Thane of Cawdor which why he says,

"There's no art

To find the mind's construction in the face.

He was a gentleman on whom I built

An absolute trust. "

Basically he means that you can't tell even what your most trusted subjects are thinking. Of course the foreshadowing is that Macbeth, who the king trusts, is planning on killing him.