"Fair is foul and foul is fair" is a theme that suggests how appearances differ from the reality beneath the surface. A good example of this is how Lady Macbeth schools Macbeth to "look like the innocent flower/But be the serpent under't."
From the time we first meet her in Act I, scene v, Lady Macbeth is concerned about the goodness in Macbeth that she perceives as weakness. She wants to alter his natural, honest behaviour to create a two-faced murderer, able to smile and shake Duncan's hand, while simultaneously plotting the King's murder.
When she first suggests the potential that Duncan be murdered in their home, she comments on how easily Macbeth's face gives his true feelings away:
Your face, my thane, is as a book, where men
May read strange matters. To beguile the time,
Look like the time.
So, Lady Macbeth schools Macbeth in how to appear "fair" while remaining "foul" underneath.
She displays her own skill in this art at the banquet in Act III, scene iv. Even when she doesn't know...
(The entire section contains 2 answers and 634 words.)