They play in the background, or are called up when the stars aren’t available. But sidemen musicians, back-up baseball players and Broadway understudies have things in common: an understanding of the importance of their role and talent, and a humility that allows them to shine even when they’re not number one. Correspondent Jim Axelrod talks with Billy Joel sideman Mark Rivera; catcher Erik Kratz, who spent 11 seasons with 14 different major league organizations; and Kayla Pecchioni, whose Broadway performances are often stepping into the shoes of the star.