BUENOS AIRES, Argentina -- At first glimpse, Gael Garcia Bernal's boyish looks don't bring revolutionary icon Ernesto "Che" Guevara to mind: no goatee, no military beret, no fatigues. That's a good thing, however, as the 26-year-old rising Mexican star redefines the leftist revolutionary for the big screen. The Motorcycle Diaries, a Robert Redford-produced accounting of Che's youthful escapades across South America, opened Thursday in Guevara's native Argentina and is coming in September to American audiences. It has screened in Cuba to enthusiastic audiences. Garcia Bernal, who previously starred as Julio in Y Tu Mama Tambien, has a seemingly impossible task:...