Actor, and in the series a piece of evidence rather than a subject. Bob uses him to support the claim that McLuhan was the greatest actor of his day: Nicholson credited McLuhan's cliché book as an insight and, in a 1985 interview, said the script he wrote for Head was based on McLuhan's theories. Start with dialogue 73.
Across the dialogues
Recruited as evidence for the actor thesis — a working Hollywood star crediting McLuhan turns "the global theater turned everybody into actors" from a figure into a documented transmission
The transmission is documentary rather than inferred — a screenplay is attributed to McLuhan by its author, which is what lets Bob argue the influence reached the performers themselves and not only their critics.
"And he said the script he wrote was based on the theories of Marshall McLuhan."
The scene supplies the mechanism of transmission — not a reading list but a household, with the book circulating socially among actors through a McLuhan family connection.
"And Mary McLuhan used to hang out at David Greenberg's place in Pasadena where Jack and Warren Beatty would show up after going to the beach where they would read Understanding Media to each other."
Connections
Nicholson credited McLuhan's cliché book as a major insight, and the Head script to his theories.