The use of pronouns might seem random in Shakespeare’s plays, and it is sometimes, perhaps often, the case that usage depends on the actor or prompt who wrote down or marked the original script. However, it is useful to note that there were subtle class and social distinctions between apparently ordinary or neutral forms of address. In her edition of Twelfth Night for the Arden Performance series, Gretchen E. Minton (who is also dramaturge for Montana Shakespeare in the Parks) includes a brief discussion of the difference between ‘You’ and ‘Thou’:

Twelfth Night depicts a society that is extremely class-conscious, and as in other Shakespearean plays, the forms of address offer important clues about hierarchies and status. The formal pronoun ‘you’ is habitually used by social superiors when addressing one another (e.g. Olivia and Orsino), and when others address them, while the familiar pronoun ‘thou’ is used when characters address social inferiors, such as the Captain, Feste and servants.

The moments when characters break this protocol signify shifting relationships or challenges to power structures. Sir Toby is especially adept at quick shifts between pronouns, thus he counsels Sir Andrew in his usage when writing the challenge letter to Cesario: ‘If thou “thou’st” him some thrice, it shall not be amiss’ (3.2.55-7). In this situation the familiar pronoun is intended to draw attention to class difference, thus provoking Cesario to seek satisfaction for being addressed with a term of contempt. Feste’s breaking of the expected linguistic code serves quite another purpose; he daringly uses ‘thou’ with Olivia: ‘why mourn’st thou?’ (1.5.71) he asks her, questioning her excessive show of grief for her brother. Feste’s familiarity is allowed because as a fool he is licensed to be witty, and even to question the actions of his superiors.

The familiarity of ‘thou’ can imply the low status of a servant, thus Sir Toby relishes the opportunities to employ this pronoun when addressing Malvolio. Recognizing the importance of her steward, however, Olivia invariably uses the formal ‘you’ when addressing Malvolio. Given such a precedent, Malvolio is thrilled to see that he is addressed as ‘thou’ in the letter he believes Olivia wrote to him; like the word ‘fellow’ (3.4.92ff), such a pronoun signifies for Malvolio Olivia’s desire to move beyond a professional relationship.

Although Malvolio is sadly mistaken, other relationships do signal intimacy through the familiar pronoun. Sir Toby and Maria address one another as ‘thou’, indicating the close relationship that they share. Olivia is polite with Cesario and Sebastian, using ‘you’ as the form of address, but in her first appearance after marrying Sebastian, she telling calls him ‘thou’.

Gretchen E. Minton, ‘Introduction’ to Twelfth Night, Arden Performance Editions, London and New York, 2020, pp.35-6.