Cesario's Appearance
When Orsino first meets Cesario (Viola in disguise), he launches into an elaborate description of "his" face, focusing on the fact that he has rather feminine features. This is one of the first examples of dramatic irony in the play, as the audience is well aware that Cesario is really Viola and that the allure Orsino feels for Cesario is really sexual attraction.
Olivia's Love
Despite swearing off men for seven years to mourn her brother's death, Olivia almost immediately falls in love with Cesario. In another moment of dramatic irony, the audience knows that Olivia is also attracted to Cesario's androgynous appearance and behavior, like Orsino, and that she has actually upheld her vow not to pursue a man because she is in actuality pursuing a woman.
Malvolio's Social Ascent
When Malvolio receives a letter from Olivia (forged by Maria) expressing her affection for him, he immediately sees it as an opportunity to climb the social ladder and better his station. He is not so much concerned with Olivia herself as he is with the luxury and power that having her as a wife will bring to him. Of course, the audience knows that the letter is simply part of Maria's plan to humiliate Malvolio, who suffers a cruel fate at the end of the play.
Sebastian's Arrival
Although absent and presumed dead for a large part of the play, Viola's twin brother Sebastian arrives in Illyria and is immediately assumed to be Cesario. It is not until he and "Cesario" are in the same room that the truth is revealed. Until that point, however, the audience watches as characters who had previously interacted with Cesario now approach Sebastian, who, new to the town, has no idea what is going on.
