Appearances are often used against people in Macbeth, as people appear innocent when they are really plotting against others, and the impossible becomes possible.
Macbeth seems to be a brave, noble, and loyal subject of King Duncan. Lady Macbeth tells Macbeth that he should pretend to be Duncan’s friend.
[Bear] welcome in your eye,
Your hand, your tongue; look like the innocent flower,(70)
But be the serpent under't. (Act 1, Scene 5, p. 20)
While Macbeth and Lady Macbeth welcome Duncan into their home, they are secretly plotting to kill him. He has no idea how much danger he is in. He thinks that Macbeth remains loyal to him.
In Macbeth, the impossible becomes possible. Macbeth assumes he is invincible because he cannot be killed by a man born of a woman, and he is safe until Birnam Wood comes to Dunsinane.
Macbeth shall never vanquish’d be until
Great Birnam Wood to high Dunsinane Hill
Shall come against him. (Act 4, Scene 1, p. 60)
Macbeth therefore assumes he is fine. After all, how can a forest come to his castle? He does not realize that Malcolm’s army is going to chop down trees to hide. Reality is hidden in Macbeth’s pride and what seems possible.
Macbeth also assumes that no man can kill him. All men were born from women, right? Actually, it turns out that Macduff was born by C-section.
Macduff was from his mother's womb
Untimely ripp'd (Act 5, Scene 8, p. 88)
As it turns out, Macbeth was too confident. He believed what the witches told him. He felt safe. He did not foresee the technicalities that will catch up to him in the end.
In Macbeth, appearances often mask a disturbing reality. Things are not always what they seem.