The actor - who plays Johnny Cash in the new biopic - tried to back out of the chat, because he feared vomitting on TV.
Oprah, who is celebrating the 20th anniversary of the show, tells US TV show Extra, "Nervous is not the word. He was a wreck. I've never seen nervous like that before.
"I had to go in the green room, and literally said to him, 'You don't have to put yourself through this.' He said, 'I think I'm going to throw up.'"