FESTE I am indeed not her fool, but her corrupter of words. (3.1.8)
Feste's assertion sums up nicely his relationship with Olivia. His job as a licensed fool not only involves entertaining the Countess, but also pointing out when Olivia's behavior or speech is silly or foolish. (Like when Olivia calls him a "fool" and Feste shows her that Olivia is the real "fool" if she continues to mourn for her brother instead of engaging with the world around her.) Also, Feste's use of the term "corruptor" recalls his association of wordplay with "wanton" women. (See 3.1.4 above.)
MALVOLIO Good fool, as ever thou wilt deserve well at my hand, help me to a candle, and pen, ink and paper: as I am a gentleman, I will live to be thankful to thee for't. (4.2.11)
It's somewhat ironic that the imprisoned Malvolio begs for pen and paper to write a letter to Olivia. (Remember that a forged letter is the very thing that landed Malvolio in the dark room he seeks to escape.) This seems to be in keeping with the play's notion that written words can be both dangerous and liberating.