As in most comedies, William Shakespeare's Twelfth Night extensively uses disguises, masks and mistaken identities to add to the comical nature of the play. Viola's disguise as Orsino's page, Cesario, becomes crucial to the action in the play. Without this important element, the action in the play would slow down dramatically, making the story much less intriguing. In addition to making the play less interesting, the disguise is also necessary to develop the storyline involving Sebastian, and the confusion that his return creates. It also is vital to the conflict between Olivia and Orsino, which depends on Viola's disguise to keep things exciting.
Viola's disguise becomes increasing more important as the events take place. The majority of the plot lines depend on the disguise. Without it, the main theme of the play would be the gulling of Malvolio. In a play where most of the characters fall in love with each other, blind to the gender and true identity of the objects of their desires, a disguise like Viola's becomes the center of the action, and causes almost all the of the important aspects of the play.
The confusion that Sebastian creates when he returns would not occur without Viola's disguise. Sir Andrew believes that the woman of his desires, Olivia, is spending too much time with Cesario, and challenges him to a duel. As he put it, Olivia was doing "more favors to the Count's servingman than ever she bestowed upon me." (3-2 l.5-7) At first, Viola is nearly forced into a battle, but is saved when the confused Antonio arrives. Later on, Sebastian and Andrew do get involved in a scuffle, for which Viola is unjustly blamed. Finally Sebastian and Viola are reunited, but only after they have already caused a large amount of chaos and have confused everyone. It is only then that everyone begins to discover the extent of Viola's trickery.
More disorder is created when Olivia, who Orsino is hopelessly in love...