Who is Vin Marbad, Mack Austin, Neeli Zutnick, Corbell Veeker from Hollywood: A Novel and what is their importance?

The author includes a number of characters in the novel, who play an important role in a chapter or two, but then disappear. These characters are for the most part not directly connected to the more publicly viewed aspects of Hollywood filmmaking, and do not have a clear and/or as obvious a counterpart to some of the more celebrity-identified characters. Thus, it is reasonable to assume that these characters (accountant Marbad, director Austin, attorney Zutnick, photographer Veeker - all intense, all demanding, all to one degree or another self-important) are themselves portraits of real-life individuals.