Few moments in cinematic history are as unexpectedly electrifying as James Cagney’s spontaneous tap dance down the White House stairs in “Yankee Doodle Dandy” (1942), a moment that wasn’t in the script but remains one of the film’s most iconic scenes. Cagney’s bold, stiff-legged dancing style throughout the film was not his own invention; it was a carefully studied imitation of Broadway legend George M. Cohan, the larger-than-life composer, performer, and patriotic showman whose life story forms