By the time this film was made (from November 1967 to December 1967), Boris Karloff was 80 years old and in very poor health, suffering from emphysema along with rheumatoid arthritis, had only half of one lung and spent the time between takes in a wheelchair with an oxygen mask on. He also wore braces on both legs and had difficulty standing or walking without his cane; the weakness of his legs is visible in some scenes in the film. Fortunately, Karloff lived long enough to view the completed film as well as enjoy the well-deserved accolades he received for this performance.
Roger Corman told director Peter Bogdanovich he could make any film he wanted to, on two conditions: he had to use stock footage from the film The Terror (1963), and he had to hire Boris Karloff for two days (Karloff was under contract and owed Corman those two days). Karloff was so impressed with the script that he refused pay for any shooting time over his contracted two days. He worked for a total of five days on the film.
As the film includes extracts from Roger Corman's film The Terror (1963), Corman suggested to director Peter Bogdanovich that the little-known actor who appears opposite Boris Karloff in that film should also appear in this one, perhaps in the role of the killer. Bogdanovich turned down this idea. The young actor later became world famous shortly after this film first appeared: he was Jack Nicholson.
The freeway shooting scenes in the film were "stolen", meaning they were done without permits. Walkie-talkies were used to communicate with the cast members out on the freeway, telling them when to act as if they had been shot. Two cameras were used - one with a telephoto lens and one with a wide-angle lens. The last shot was done with the woman trying to run away as the police had been called after someone saw her fall, and the production crew bugged out before they arrived.
Director Peter Bogdanovich planned to have Boris Karloff actually appear in the film for about 20 minutes with two days of filming, and have the stock footage from the film The Terror (1963) add another 20 minutes of screen time for Karloff. In the final film, Karloff is actually on-screen (not counting the scenes from "The Terror") for about 30 minutes and shot all of his scenes in five days.