request that a person with the letters MOAI in their name act a bit strange by dressing in yellow stockings for Olivia. After convincing himself that he is in fact the addressee, he immediately seeks to fulfill Olivia's wishes. Continuing the prank, Maria and others note to Olivia that Malvolio is acting strange. By Act 5, he confronts her with the letter and says:

Alas, Malvolio, this is not my writing,Though, I confess, much like the character.But out of question, 'tis Maria's hand.

This proves that Malvolio was a victim to self-deception. Beyond the fact that the letter doesn't even mention his name, it's not even Olivia's handwriting. He allowed himself to believe that a relationship between himself and Olivia was actually possible, when, in reality, it wasn't. 

A less obvious example of self-deception could be the way that Olivia portrays herself in the beginning of the play. As she mourns the death of her father and brother, she swears that she will be in mourning for the next seven years and will not allow any man into her heart. This is particularly addressed to the Duke, who has been pursuing her for quite some time. However, upon seeing Cesario's (Viola's) face, Olivia notes:

Methinks I feel this youth's perfectionsWith an invisible and subtle stealthTo creep in at mine eyes. Well, let it be.

In other words, she is going to willingly allow Cesario's good looks to be the object of her affection - a complete 180 degree shift of how she responded to the Duke's pursuits. In other words, she may have been using the mourning as a disguise as to why she can no longer be in love. 

This theme of self-deception connects to a larger theme of lies and deceit in the play. There are obvious examples of disguise to try and accomplish personal interests; however, those disguises do not let the characters reveal their true selves. That's why these cases of self-deception are particularly interesting. 

The theme of self-deception can be seen all throughout Twelfth Night. Two examples can be seen with respect to both Olivia and Duke Orsino.

Olivia deceives herself into believing that her excessive, prolonged grief over her brother is the acceptable and right thing to do. We learn in the very first scene from Valentine, one of Duke Orsino's servants, that Olivia plans to mourn her brother's death for seven year, to never show her face in public unless she is wearing a veil, and to cry every single day, as we see in some of Valentine's lines, "The element itself, till seven years' heat, / Shall not behold her face at ample view" (I.i.27-28). We also learn later from the sea captain who rescued Viola that Olivia is rejecting the presence of any company in her home except for her own servants and family members. Even widows only mourned for one to two years, and during that mourning period, they are certainly permitted to abstain from society. However, Olivia is a sister and not a widow; therefore, not only would a seven-year mourning period be excessive for a widow, it is certainly excessive for a sister. Feste points out her self-deceptive, foolish behavior when we first meet him in Act 1, Scene 5. When Olivia demands that he be taken away because she is angry with him for having been gone from the house so long, Feste turns the tables and asserts that she is the true fool. He asserts it by saying that it is foolish for her to mourn so over a brother whose soul is in heaven rather than in hell.

Duke Orsino is also guilty of self-deception with respect to what he thinks is his love for Olivia. The sad reality is that he is not really in love with her so much as he is in love with the idea of loving her. He doesn't really know Olivia; he has never spoken with her; he has only fallen in love with her beauty. He also believes that her grief over her brother shows just how much she would love a man should she fall in love, which makes him continue to pine for her. However, his love for her is really more of an obsession, showing us that he is deceiving himself into believing that he is in love with her.

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.