The piece the "host" is playing at the piano in the scene between Anthony Hopkins and Jeffrey Wright is called "Reverie" by Claude Debussy. The reveries are also an important part of the plot.
At some point, Ford talks about the bicameral mind. This is a reference to Julian Jaynes' groundbreaking book, "The Origin of Consciousness in the Breakdown of the Bicameral Mind." Jaynes' basic idea was that the corpus callosum, that bundle of nerves that ties together the two hemispheres of the brain, is a recent development. Prior to that, the only way the logical, rational side of the brain could communicate with the emotional, instinctive side was through auditory hallucinations, which primitive peoples interpreted as the 'voices of the gods'. The one of many examples given is of "The Iliad". Characters in the Iliad don't have any inner dialogues with themselves, there is no planning or foresight, and all their actions are undertaken from instructions given to them by the gods. As society and its situations became too complicated for this process to be effective, the corpus callosum developed and true self-consciousness arose.
A day at the park costs 40k, said by William's brother-in-law.
Ford is depicted as a younger version of Anthony Hopkins. For this to be done, effects supervisor Jay Worth collaborated with ILP, a Stockholm-based VFX studio. References were pulled from Hopkins' early acting career, when he was in his 30s and 40s.
In Robert Ford's (Anthony Hopkins) office, one of the grey faces mounted on the wall behind him - in the upper right-hand corner - is the death mask of Abraham Lincoln.