Symbolism, Imagery, Allegory
Somebody forgot to tell the characters in Twelfth Night that it's rude to pass notes in class. Yes, we're aware this joke may not hold the same meaning for you as it does for those of us who experienced junior high without unlimited text-messaging. But seriously, what's up with all the love letters? Duke Orsino makes "Cesario" memorize one so "he" can recite it to Olivia, Maria forges a love note in order to convince Malvolio that Olivia is in love with him, and Malvolio writes one to the Countess when he's imprisoned in the dark room. (OK, that last one is less a love letter than it is a hate note accusing Olivia of being a tease, but you get what we're saying here.)
So what? Well, it seems to us that all of these letters have one thing in common. Aside from their status as messages of love, they're all associated with deception and dishonesty. First, Olivia totally rejects Orsino's love note as a contrived and insincere declaration of passion, and she's absolutely right. Orsino doesn't mean any of it (not about her anyway). The letter Maria forges to trick Malvolio, of course, is completely deceptive, and Malvolio's letter to Olivia is utterly misinformed. Written words, it seems, are just not to be trusted, especially when they're contrived. Hmm. That's an awfully funny message for a wordsmith like Shakespeare to send his audience.