David Cronenberg wrote the script in six days. He has admitted that when he converted the book into screenplay format on his computer, he realized it was so perfect that his only work was to separate dialogs from narration.
In one scene a public display shows the text, "A SPECTER IS HAUNTING THE WORLD. CAPITALISM." This is a take-off on "A specter is haunting Europe - the specter of communism." which is the opening line of the political pamphlet "The Communist Manifesto", published 1848 by German philosophers Karl Marx and Friedrich Engels.
The trivia items below may give away important plot points.
Opens with an epigraph from the poem "Report from the Besieged City" by Polish poet and dramatist Zbigniew Herbert(1924-1998), just like the 2003 novel by Don DeLillo: "a rat became the unit of currency". The poem was originally published 1983 in the collection "Raport z oblezonego Miasta i inne wiersze" ("Report from the Besieged City and Other Poems"). The novel and the film adaptation both extend the metaphor of the 'rats' into a leitmotif: Character Eric Packer says that he read a poem where "a rat became the unit of currency". Later we see the protestors use rat-like masks, puppets and dead rats as a symbolic expression of anti-capitalist criticism.
Being known as an incredibly faithful adaptation, only a few changes were made including the money currency (from yen to yuan) and the sex scene between Didi and Eric, which in the book takes place in a flat.