In Twelfth Night Act 5 scene 1, how is the letter that appears in this scene different from previous messages and letters in the play? What function does the letter serve, in terms of plot development?
The letter that Fabian reads out loud to the characters, which Malvolio wrote about what he has suffered, is different from the other letters in the play because it is actually based on truth, and it is a letter that was genuinely written by the character who is said to have written it. This is of course different from the letter that Malvolio receives from Maria, which is supposed to be from his mistress, Olivia. Note how the language of the letter is also very different:
By the Lord, madam, you wrong me, and the world shall know it: though you have put me into darkness and given your drunken cousin rule over me, yet have I the benefit of my senses as well as your ladyship.
There is no artifice or complex, flowery language. By contrast, the writer of this makes his point clearly and simply: what he has suffered, which he believes to be at the hands of his mistress, is intolerable and he is determined to have his revenge. The letter is therefore different because of its language and also that it is authentic. Its purpose is to sow a discordant note into an otherwise happy denouement, as the plot against Malvolio comes to light and Malvolio himself appears to state he will have his "revenge" on everybody.