When choosing a quote spoken by Duke Orsino, what you first want to do is figure out what you want to say, prove, or argue with the quote concerning Duke Orsino. To do that, it is first helpful to figure out exactly why he is important. Orsino illustrates many different themes found in Twelfth Night. The most significant theme he illustrates concerns the nature of love. He speaks of the cruelty of love, the constancy of love, and even the fickleness of love. In fact, despite his obsessive devotion to Olivia, at the end of the play, he very quickly forgets Olivia and falls in love with Viola instead, which illustrates one of Shakespeare's main points concerning the foolishness and fickleness of love. Shakespeare even uses Orsino to illustrate the foolishness of human nature, especially with respect to the foolishness of courtly love. Courtly love was a fad in the Middle Ages in which a man fell deeply in love at first sight with but was cruelly rejected by the woman. In addition, his feelings for the woman completely consumed him to the point of torment, just as Orsino's feelings for Olivia have completely consumed him (eNotes, "Character Analysis: Duke Orsino"). Hence, one thing a person can use Duke Orsino quotes to prove is that Orsino is used to illustrate one of four themes:

• the foolishness of love
• the foolishness of humanity
• the constancy/fickleness of love
• the cruelty of love

If we take the theme of the constancy/fickleness of love as our example, one good place where Orsino illustrates the problem of fickleness is in Act 2, Scene 4. In this scene, Orsino advises Viola as Cesario to love like Orsino, should Cesario ever fall in love, because Orsino is the truest of lovers, and just like "true lovers," Orsino is fickle and faithless to anything but the one he loves, meaning constant to only Olivia, as we see in his lines:

For such as I am all true lovers are,
Unstaid and skittish in all motions else,
Save in the constant image of the creature
That is beloved. (II.iv.17-19)

However, further in this scene, when Orsino next finds out that Cesario loves one who is older than he is, Orsino advises him to find a younger woman because "[men's] fancies are more giddy and unfirm ... / Than women's [fancies] are," meaning that men have a tendency to be fickle and change their minds about the women they love much more than women tend to be fickle. Therefore, on the one hand, Orsino is declaring himself to be constant, but on the other hand, he confesses that all men are fickle, and we see him prove the fickleness of men when he so quickly changes from loving Olivia to loving Viola at the end of the play.