Fun article from the New Yorker about understudies; mentions Jon Rua's recent turn as Hamilton:
The Thrilling Uncertainty of the Understudy
An understudy is not usually cast on the strength of his or her dazzling charisma or vivid originality—that’s the lead’s claim—but for more terrestrial virtues: reliability, versatility, and the capacity to master a lot of lines without having the useful practice of enacting them nightly. That is not to say that understudies are necessarily less gifted than the stars they are called upon to cover for. Jon Rua, who is also a choreographer, could best Lin-Manuel Miranda in a dance off any day of the week. (Witness this video, which Miranda also tweeted.) And Javier Muñoz—who happened to be substituting for Miranda the first time President Obama saw the show, and who was also on stage when Beyoncé and Jay Z were in attendance—has a more mellifluous singing voice than Miranda. Muñoz has been praised for other gifts, too. Ben Brantley, reviewing his performance in the Times, wrote, “Hamilton is sexy on Sundays.”
http://www.newyorker.com/culture/cultural-comment/the-thrilling-uncertainty-of-the-understudy