By the time this film was made (from November 1967 to December 1967), Boris Karloff was 80 years old and in very poor health, was suffering from both emphysema and rheumatoid arthritis, had only half of one lung (both the other half and the other lung had been removed due to cancer) and spent the time between takes in a wheelchair wearing an oxygen mask. He also wore braces on both legs and had difficulty walking or even standing up without his cane; the weakness of his legs is visible in some scenes in the film. Fortunately, Karloff lived long enough both to see the finished film and enjoy the well-deserved accolades that he received for his performance in it.
Roger Corman told director Peter Bogdanovich that he could make any film that 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 scenes from Roger Corman's film The Terror (1963), Corman suggested to director Peter Bogdanovich that the little-known actor who appeared 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 that 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 scene was done with the woman trying to run away, as the police had been called after someone saw her fall as if she had been shot, and the film's production crew were extremely nervous 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.