A common theme in literature is not to judge a book by its cover. Analyze how Twelfth Night uses this theme. Give two examples to support the analysis.

In William Shakespeare's Twelfth Night, the issue of taking someone at face value, or "judging a book by its cover," is a pivotal one. The characters assume disguises and false names, pursuing their own needs and desires under the guise of serving someone else. This leads to an ironic and humorous love triangle, which causes the characters much grief before its resolution. Of course, there is only resolution when the characters' true identities are revealed.
The first disguise of the play, and the most important one, is that of Viola. She is a young woman who has been shipwrecked and disguises herself as a man named Cesario so that she can serve in Duke Orsino's court and find information about her missing brother, Sebastian. While disguised as Cesario, the Duke asks Viola to help him win the heart of Countess Olivia. When Olivia meets "Cesario," she sets her sights on him rather than accepting the Duke's suit. All the while, Viola falls in love with the Duke, though she is disguised as a man and cannot express her love for him.
Viola's disguise itself is a test of the characters' willingness to judge a book by its cover and often leads to ironic conversations between the characters, such as in act 3, scene 1:

Olivia: Stay; I prithee, tell me what thou think'st of me.
Viola: That you do think you are not what you are.
Olivia: If I think so, I think the same of you.
Viola: Then think you right: I am not what I am.
Olivia: I would you were as I would have you be!

Countess Olivia believes she is flirting with Cesario in this conversation, while Viola is trying to subtly hint that Olivia's flirtations are aimed at the wrong person. Olivia also reveals her stubbornness here, as she is happy to believe that Cesario is who she wants him to be and unwilling to change her perspective, no matter what Viola says.