Roger Corman did research by taking LSD himself. Charles B. Griffith wrote the first two drafts of the script—the first one was about the social issues of the sixties, the second one was an opera. Corman then hired Jack Nicholson to write the eventual screenplay. Corman encouraged Nicholson’s experimental writing style and gives between 80 and 90 percent credit to Nicholson for the shooting script in the director’s commentary appearing on the DVD of this film. Corman slightly modified the story to stay within budget.
Whilst most of the music actually used in the film was by Mike Bloomfield’s Electric Flag, it is interesting to note that early visuals (e.g. the band in the club at the start of the film) are of Gram Parsons and the International Submarine Band, one of the earliest country-rock bands. It had been Fonda’s original intention to use the ISB’s music on the soundtrack but, in the event, their contribution was deemed insufficiently “psychedelic” or trippy to warrant inclusion and the Bloomfield/Buddy Miles/Nick Gravenites Electric Flag is what is actually heard in the film.