While many characters are guilty of deception, the reality is that no one character could have been successfully deceived if self-deception had not also been a dominant factor. Each character who has been deceived has just as equally deceived himself or herself. One example of self-deception can be seen in Olivia.
Olivia allows herself to be deceived into believing that Viola is a man named Cesario, despite the fact that her looks make it very unlikely for her to easily pass as a man. As Malvolio describes when Olivia first asks "what manner of man" the messenger Cesario is who refuses to leave her gate until he has spoken with Olivia, Cesario looks too underdeveloped to be called a man and yet too old to still be called a boy (I.v.143). Malvolio likens Cesario to unripe fruit, even pointing out that the has no Adam's apple. Yet, by the end of this scene, Olivia has been impressed by both Cesario's looks and his bold and forthright manner of scolding her for abusing Duke Orsino's love. We see just how enamored with Cesario's looks she has become in her description to herself after he leaves:
'... I am a gentleman.' I'll be sworn thou art;
Thy tongue, thy face, thy limbs, actions, and spirit,
Do give thee five-fold blazon. (I.v.279-81)
These lines show us that she has fully deceived herself into believing Cesario really is a man despite the fact that his looks can say otherwise. Feste was not so easily duped and knew all along that Cesario was really a woman masquerading. In addition, Olivia's self-deception into loving Cesario is even likened to a madness. She states in the last few lines of this scene that her mind is at odds with the fact that she thinks Cesario is beautiful, showing us that she has both deceived herself and is likening love to a state of madness.