Note to Twelfth Night, 5.1.150, "Be that thou know'st thou art, and then thou art / As great as that thou fear'st"

Olivia tells Cesario that he is just as good as Duke Orsino—that if he asserts his status as her betrothed husband, he is therefore a Count (as she is a Countess) and has nothing to fear from a Duke (Orsino).

Editors generally assume that Olivia assumes that Cesario is denying that he is her betrothed husband because he is afraid of Orsino. However, "what thou fear'st" may simply be the status of being Olivia's husband. Olivia may be assuming that Cesario doesn't want to be a social climber. (One of the points of the practical joke played on Malvolio is that social climbing is odious.)

Imogen Stubbs as Cesario and Helen Bonham Carter as Olivia