Roger Corman also took LSD before starting the film, figuring he couldn't make a film about LSD without trying it himself. He had a good experience, and had to ask others what a "bad trip" was like in order to incorporate it into the film.
According to Bruce Dern, Jack Nicholson wrote the part of John for himself. Roger Corman, however, would not agree to Nicholson playing the role and chose Dern instead.
Paul Groves: [Holding an orange up to the horizon]
That's the sun in my hands, man! Oh, it gives off an orange cloud of light that just flows right out over the sea! Wow!
Max: I wish there was some hip way of telling you this, baby, but, ah... you're one with and part of an ever-expanding, loving, joyful, glorious, and harmonious universe.