There was a Gap commercial in the 1990s that performed "Cool"—a song and dance borrowed from West Side Story—that did fierce battle with my memory of both the original number in the movie and that film's overall energy and spirit. Guess who won? The dancers in the Gap ad were mighty fine, but I swear I could sense them counting time in their heads as they went through the steps, trying very hard not to wear their concentration like a mask.
Not so in the 1961 movie, particularly in the "America" number. The heart feels, the body follows and the mind does not intrude. Simply put, this is one of the most exuberant dance numbers committed to film. Choreographer Jerome Robbins, who co-directed with Robert Wise, rehearsed dancers for three months prior to shooting, and it shows, resulting in a sequence that builds and climaxes as if by providence. It's a fine example of dance as second nature, a thrilling showcase for freakishly talented folks with music in their bones.