Disguise, gender, and identity
The plot of Twelfth Night depends on a love triangle: the noblewoman Viola (disguised as a boy named Cesario) loves her employer, Duke Orsino; Orsino, in turn, loves the Countess Olivia and sends "Cesario" to woo her by proxy; Olivia spurns the Duke's suit and falls in love with "Cesario" instead. And this is just the main plot.
The lovers in Twelfth Night are at cross-purposes. They not only create comic misunderstandings but also challenge the stability of appearances, gender roles, and the "off-limits" territory of same-sex-desire. With male actors playing women (one of whom, Viola, is disguised as a boy), the notion of fixed gender identity may seem as transient as a disguise.