In Shakespeare's Macbeth, an important, yet underlying theme is that of appearance versus reality. Appearances are just what they are, appearances, nothing more and nothing less; what those appearances appear to be are shown through the concept of reality.
Appearances can be deceiving, even that of someone that is trusted. After receiving a report on the execution of the old Thane of Cawdor, King Duncan 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. (1.4.13-16).
Duncan is commenting on the fact that he trusted the Thane of Cawdor absolutely, and had no idea the Thane would become a foul rebel. This also applies to the feelings that Duncan has toward Macbeth although he has or will never outright state it. Trust is a part of one's appearance; the reality of it is the trusted person's actions. This statement has a dual purpose. The main purpose is to show that Duncan cannot read people. He has and will do this several times- one of them having already occurred, that being the Thane of Cawdor's deception and another yet to come, that of the deception of one of Duncan's closet subjects, Macbeth. The secondary purpose of the line is to foreshadow Duncan's demise due to his easily trusting nature. That easily trusting nature proves to be a drawback for not only Duncan, but also his subject, Macbeth.
Malicious acts, such as those of murder, can be covered up with a welcoming and pleasing appearance. Shortly before Duncan's arrival to Macbeth's castle, Lady Macbeth urges her husband to: "look like the flower/But be the serpent under't" (1.5.65-66). Lady Macbeth basically wants Macbeth to look fair, the better to hide his foul intentions. Every human being needs a sense of security, which is mankind's greatest downfall. The statement provides that sense of security with a welcoming face and presence that Macbeth's guests subconsciously want and need.
