Dennis Hopper and Peter Fonda did not write a full script for the movie, and made most of it up as they went along. They didn't hire a crew, but instead picked up hippies at communes across the country, and used friends and passers-by to hold the cameras, and were drunk and stoned most of the time.
Peter Fonda wore the Captain America jacket and rode his chopper a week around Los Angeles before shooting began, to give them a broken-in look, and to get used to riding the radically designed bike. The American flag on the back of the jacket, and on the gas tank of the bike, caused him to be pulled over several times by the police.
Dennis Hopper was going through a very bad time during production (something he later put down to marijuana not being his "creative drug of choice"). He was in a state of drug-induced paranoia and he screamed at everyone. Crew members secretly recorded his tirades and sent the tapes to the production company in Los Angeles to explain why so many of them quit the film.
The film's driving sequences were among the first to deliberately use of lens flare to add atmosphere. Initially derided because they had failed to keep the shots 'clean', the technique went on to become commonplace in cinematography.
During Jack Nicholson's "UFO" speech, Dennis Hopper was intent on getting him very stoned on marijuana. The laughing that eventually broke up his speech was not planned, and when Nicholson repeats the line "it . . . it . . . would be devastating . . . " it was the next take. However, Nicholson was largely able to stick to the script as written, much to the crew's amazement. This is a stark contrast to the other cast members, who improvised most of their lines.
Bridget Fonda, Justin Fonda: Children of Peter Fonda appear as hippie commune children, along with their mother, Susan Brewer.