Producer Rubin ended up signing Cash to his American record label and is widely credited with revitalising the fallen star's career.
But he admits he was devastated to see patrons ordering food as they watched Cash perform at the Orange County, California, venue.
Rubin, who met Cash backstage that night, says, "As he was singing you could sit there and order your meal. I think many people thought his best work was behind him.
"But when I got there, I started to feel nervous. He was the Man In Black. I guess it was the mythology."
Cash died in 2003 aged 71.