What does Duncan say about physiognomy?

Duncan says of the traitor the Thane of Cawdor whom he has ordered executed:
There's no art
To find the mind's construction in the face:
He was a gentleman on whom I built
An absolute trust.
Shakespeare himself did not necessarily believe what he has his character King Duncan say. In The Tempest, for example, Shakespeare has the character Gonzalo say something quite different during the big storm in the first scene of the first act.
I have great comfort from this fellow. Methinks he hath no drowning mark upon him; his complexion is perfect gallows. Stand fast, good Fate, to his hanging. Make the rope of his destiny our cable, for our own doth little advantage. If he be not born to be hanged, our case is miserable.
Gonzalo is speaking of the Boatswain, who has just been insolent. Later Gonzalo will repeat his prediction:
I'll warrant him for drowning, though the ship were no stronger than a nutshell and as leaky as an unstanched wench.
Gonzalo is assuming that if the ship sinks they will all necessarily be drowned together, so if the Boatswain survives to be hanged later on, as his face indicates to Gonzalo that he must, then they will all be saved from drowning.
Does an innocent and kindly face mean that a person has an innocent and kindly nature? Does a brutal face mean that a person must be brutal? It is an interesting speculation. We tend to judge people by their faces, and particularly by their expressions. But some wicked people are clever and skilled enough to look benign. Claudius in Shakespeare's Hamlet is always smiling although he is a sinister person with a guilty conscience. Hamlet says of Claudius in Act 1, Scene 5 of that play:
O villain, villain, smiling, damned villain!
My tables--meet it is I set it down
That one may smile, and smile, and be a villain; (I.5)