What would Bartleby be without his catchphrase? "I would prefer not to" really sums up the whole enigma that is Bartleby. If we examine this phrase, we see that it's kind of an odd thing to say – it's not "I won't," or even "I don't want to." "Prefer" indicates something a little more subtle…Bartleby would, presumably "prefer" to be doing something else, if he had his druthers. Even more interesting is the way in which this catchphrase infects all the other people in the office, as the Narrator exasperatedly notes (67-68). Melville demonstrates the powerful effect that language can have simply by using this same formulation – "I would prefer not to" – over and over (and over) again.
Interestingly, through the Narrator's eyes, we get an occasionally hilarious dose of rather indirect direct characterization. He describes his employees and their quirks in a euphemistic but still very precise manner; through these descriptions, we manage to glean quite a lot about both them and him. For example, the Narrator's description of Turkey exposes a great deal about both characters – he never comes out and says that Turkey is a drunkard, though obviously he is, and he also demonstrates his own weakness and fear of confrontation through his way of dealing with Turkey's problems (mostly ignoring them and making the best of things).
The names and nicknames Melville presents us with define their characters quite clearly, particularly in the cases of Turkey, Nippers, and Ginger Nut. There's something that is oddly both condescending and endearing about all of these nicknames; through them, all three of the clerks become caricatures, rather than real people. It's notable that all three nicknames are rather dehumanizing; a turkey really is an animal, "Nippers" sounds like the name of a terrier rather than a man, and poor Ginger Nut is simply named after the snacks he delivers.
The two most important characters, Bartleby and the Narrator, are separated from these three nicknamed characters – Bartleby, by the fact that he is called by his real name and nothing else, and the Narrator, by the fact that he has no name. Bartleby's moniker might indicate his stubborn refusal to be anything but himself, while the Narrator's namelessness seems to indicate a certain universality – he's a kind of Everyman, perhaps, which might be why we, the readers, identify with him.
We don't "see" all of the characters here, but what we do learn about appearance is quite helpful. Turkey's blazing red face, for example, demonstrates both his alcoholism and his hot temper. Bartleby, on the other hand, is pallid, thin, and "cadaverous," which exhibits his strange emotional coldness and inhuman remoteness.