Kristen Schaal has been added to the opening credits to reflect her promotion to full regular. Ironically, she does not appear in this episode.
Harvey Guillén is clearly wearing a fat suit to hide the fact that he lost a great deal of weight for other roles.
The book Nandor is reading, "I'm OK, You're OK" by Dr. Thomas Anthony Harris, was published in 1967 and became a bestseller in 1972, remaining on the bestseller lists for two years. It is a self-help book which promotes the idea of solving communication problems through "transactional analysis." The idea is that all communication is initiated by one of three aspects of the personality, which Harris calls the "Parent," "Adult," and "Child," (P, A, and C) and addressed TO one of these three in the person being addressed. If that person responds FROM that part of their personality has been addressed TO the part of the original speaker's personality that started the conversation, communication continues, as a power dynamic has been established and accepted by both parties. However, if one of the people speaking responds either from or to the "wrong" persona, this results in a "crossed transaction," and the only way meaningful communication can continue is if one of the people immediately switches to the appropriate persona. For example, if one person's Parent speaks to another person's Child (an attempt to assert dominance), but the second person responds with their Adult addressing the first person's Adult (refusing to be in a submissive position, but seeking to communicate as rational equals), then for there to be meaningful discussion, the first person must switch to speaking A-to-A.
While the book is much deeper than Nandor's affirming that Guillermo or what he is doing is "OK," the actual themes of the book are readily evident in their overall relationship, with all of the vampires tending to approach Guillermo as P-to-C, while Guillermo wishes to approach them A-to-A, but constantly falls back on approaching them C-to-A in order to address them at all. This kind of crossed transaction seems to be the foundation of the problems between Nadja and The Guide, as well.
When changing the settings of the sound machine with Guillermo, Nandor paraphrases a famous line from Bram Stoker's Dracula. The original reads "the children of the night. What music they make!". Nandor says "the creatures of the night. Such beautiful music they make". Nandor has been reading a lot lately and one of those books was probably Dracula.