Deceit is a main theme in Twelfth Night, since most of the characters in the play are engaged in either deceiving themselves or in deliberately, or accidentally deceiving others.

After losing her brother in a shipwreck, Viola disguises herself as a male, Cesario, and manages to deceive everyone. In this, she causes much complication to the plot of the play. Firstly, Orsino sends Her on his behalf to woo Olivia, only causing Olivia to fall in love with Cesario. Also, there are many scenes where Cesario is often mistaken for Sebastian, her twin brother. This contributed to a fight between "him" and Sir Andrew; and even a mistaken marriage to Olivia, who actually marries Sebastian, thinking that he is Cesario. The marriage got Cesario into a serious misunderstanding with Orsino, with the Duke thinking that Cesario has taken his love away from him. Not only has Viola's disguised...