Best lead chemistry in film

"Action requires reaction. It’s a chemical thing. But reaction rates differ, so we want you to don your lab coats and list the ten lead-actor pairings that create the most explosive results for you in the notorious charade of cinema. It’s secretly a Showdown in praise of casting directors, those who match the bright stars, and it’s not (all) about sex, baby—it’s also about love and basketball and keeping the bus above 50 mph. The leads might be best fiends, she might be his friend’s mother, they may be the only lovers left alive. Fools rush in, so take your time now voyager. This is a titanic (and highly subjective) task."