Shakespeare sets the mood for this romantic comedy by having Orsino speak these words at once: If music be the food of love play on, give me excess of it, that, surfeiting, The appetite may sicken, and so die That strain again! It had a dying fall. (Shakespeare I
Upon assuming a masculine appearance, Viola almost instantaneously develops a remarkable wit -- a typical masculine trait, which was most likely suppressed within her unconscious; up until now, it must have found expression via her brother. However, though perceived as a man by everyone around her, Viola is well aware of her true feminine nature, falling for Orsino (Dreher, 1986)
Intriguingly, Viola chose to conceal her femininity at the play's beginning. Her apparently-deceased brother is probably the physical manifestation of Viola's animus, akin to Lady Olivia's situation (Dunning, 2015)