Viola, as Cesario, has won Orsino's complete trust in three days and is sent to woo Olivia on his behalf. In an aside, she reveals she loves Orsino herself.
Three days have passed. Viola, dressed as Cesario, has already risen to become Orsino's most trusted attendant. Valentine describes her as more valued than anyone else in the household. Orsino calls Cesario and sends everyone else away.
He gives Cesario a mission: go to Olivia, refuse to leave until she agrees to hear him, and press his suit with all the passion of someone who genuinely feels it. He believes Cesario's youth, smooth appearance, and high voice will make the appeal more effective than any older or more obviously masculine messenger.
Cesario agrees. Orsino promises to reveal the history of his love on the way. In an aside at the very end of the scene, Viola acknowledges what the audience has probably already guessed: she has fallen in love with Orsino and wishes she were the one he wanted to marry.
This scene establishes the central dramatic irony of the romantic plot. Orsino is sending the person who loves him to plead his case with the person who will not love him. Viola must advocate for a man she loves, to a woman who will love her disguise, on behalf of feelings she cannot declare. Every instruction Orsino gives her tightens the situation she is already in.
Orsino's physical description of Cesario is worth examining. He says Cesario's lip is smoother than Diana's, his voice is like a maiden's, his appearance is all but entirely feminine. His description is accurate, though he does not know why. He is, without realising it, describing the woman he will eventually marry while believing he is praising the androgynous beauty of a boy. The scene makes Orsino's later attraction to Viola feel earned in retrospect: he has been responding to her presence throughout.
Viola's aside, "Whoe'er I woo, myself would be his wife," is the first time she is given direct access to the audience. It is brief and matter-of-fact. She does not dwell on the impossibility of her situation or perform anguish about it. She names what she feels, accepts that it creates a problem she cannot immediately solve, and continues. This restraint is characteristic: Viola does not dramatise herself the way Orsino constantly does.