Robert Downey, Jr. wrote most of his lines down on post-it notes and scattered them around the set so he could read off them while filming a scene. The rotoscoping team simply animated over the notes to remove them from the film during post-production.
According to Writer and Director Richard Linklater, filming was completed in twenty-three days. The animation took eighteen months.
When Bob Arctor (Keanu Reeves) sits on the stage waiting to give his speech to the Brown Bear Lodge, one of the images his scramble suit displays is Philip K. Dick. This is a clever reference to the novel, in which the scramble suit is said to show the likeness of its creator once in every several million permutations.
(At around fifty-five minutes) When Charles Freck (Rory Cochrane) goes to the liquor store to buy wine, one of the brand names being advertised is St. Ubik. This is a reference to Philip K. Dick's novel "Ubik".
Philip K. Dick's daughters gave Writer and Director Richard Linklater their father's personal copy of the novel "A Scanner Darkly" when he completed this movie.