Before playing Lt. Col. Bill Kilgore in “Apocalypse Now,” actor Robert Duvall, a U.S. Army veteran, persuaded a Vietnam War helicopter pilot to explain the realities of air cavalry life.To portray Augustus McCrae in “Lonesome Dove,” Duvall studied equestrian with a champion show jumper and befriended West Texas football legend Sammy Baugh, mastering his bowlegged walk and slow dr...