NEW YORK (AP) — Rosanne Cash believes her father, Johnny Cash, didn’t get into amphetamines to get high. She says it started because “he was trying to do his job.” Her father’s drug addictions and failed marriage to Rosanne’s mother, Vivian, are chronicled in the two-hour documentary, “Johnny Cash: American Rebel.” His triumphs are included as well, like how music producer Rick Rubin revitalized Johnny’s career. Rosanne Cash says she remembers thinking Rubin’s involvement was another ridiculous idea, until she saw how well they worked well together and how they loved each other like brothers. “Johnny Cash: American Rebel” premieres Saturday on CMT.