Shakespeare uses disguise in his play, Twelfth Night, to cause confusion and internal conflict between his characters and it is this confusion and conflict that appeal to the audience. It keeps them wondering how many more of these situations will arise, and in the end, how will this confusion and conflict be resolved?
The first time that this is evident is in Act I, Scene IV, where Cesario, really Viola is sent by her master, Orsino, to win the love of Countess Olivia for him. At first it seems as if nothing is out of the ordinary, but Cesario throws a spin on things with his last words of the scene. Cesario indicates that he will do his best to win over the lady, but then in an aside says "Whoo'er I woo, myself would be his wife." (I. IV.41)
This makes things much more difficult. What will happen when a messenger who loves his master is sent to win over the love of the one his master desires? This is a case where Shakespeare's use of disguise has left the audience in suspense. The audience is left waiting to see how this matter will play itself out. Little do they know, that disguise will play another important role, in muddling this problem even further.
In the next scene, Act I, Scene V, Cesario will arrive at the home of Countess Olivia, and after some difficulty, will eventually be granted an audience with the Countess. Cesario is very eloquent, almost relentless, in expressing what love that the Duke Orsino has for Olivia, but Olivia rejects the Dukes offer, saying that she cannot return his love. This is where things really get perplexing.
Once Cesario exits, Olivia reviews what Cesario has said, and begins to think of the messenger. At this point, the audience realizes what has happened, Olivia has fallen for the messenger. Even this comes as a surprise to her, she says "Even so quickly may one catch the plague?"(I. IV.281)
To stand back and see what has arisen, all because of Viola's disguise, is overwhelming and...