Jon Rua stepped into the title role in “Hamilton” on Sunday. Walter McBride / INFphoto.com via Corbis

For close observers of the cultural phenomenon that is “Hamilton,” a new milestone in the musical’s unfolding journey was reached last week: the appearance of the understudy in the title role. “Today marks the premiere of @JonRua as Hamilton. Go get ’em Mano. You got this,” Lin-Manuel Miranda, the show’s star, author, composer, and social-media maestro, tweeted on Sunday, little more than an hour before the curtain was to rise on the matinée.

Rua wasn’t replacing Miranda, exactly; for Sunday matinées, the role of Hamilton is regularly played by another actor, Javier Muñoz, who has served as Miranda’s alternate for the show’s Broadway and Off Broadway runs. But on this occasion, both Miranda and Muñoz were indisposed. Rua had been prepping for the possibility of taking on the role; as a member of the “Hamilton” ensemble, he usually plays the small part of General Charles Lee. He is also one of the show’s exhilarating dancers. In a backstage video, posted to Instagram on Sunday afternoon by Leslie Odom, Jr., who plays Aaron Burr, Rua is seen being circled by a huddle of cast members, serious and focussed, then casting a smile at Christopher Jackson, who plays George Washington: a foot soldier being called into extraordinary service.

Theatre history offers celebrated instances of understudies who went on to become superstars. There’s Shirley MacLaine, whose break came in 1954, when she was temporarily promoted from the chorus of “The Pajama Game” to cover for Carol Haney, the show’s star, who had sprained her ankle during a performance. “I had worked for this moment since I was two,” MacLaine said later. As the artistic director of Britain’s National Theatre, Laurence Olivier cast a young Anthony Hopkins to be his understudy as Captain Edgar in August Strindberg’s “The Dance of Death.” When Hopkins was obliged to step in, Olivier later wrote, he “walked away with the part like a cat with a mouse between its teeth.” But despite those inspiring examples, the job of the understudy is often a thankless one. Understudies suffer the artistic frustration of not being able to make a role entirely their own, even should they have the opportunity to perform. They bear the difficult knowledge that their own opportunity for success depends upon the chance of someone else’s misfortune; possibly, they even wish for it. “I secretly wanted to poison the lead I was understudying,” as a columnist for Playbill has put it.

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.”

Even so, an understudy is an insurance policy that the producers hope not to have to call upon. Audiences who feel they have paid for a star and expect to see one can be outspoken in their dismay at the appearance of a substitute. “I could feel these waves of hatred coming off of the audience,” Bebe Neuwirth, the Broadway star, said, when interviewed for the documentary “Standby,” of her first outing as an understudy—for Debbie Allen, in “Sweet Charity,” in 1986. Understudies are their own beleaguered special interest: few ordinary theatregoers think about them until they are forced to. Like all beleaguered special interests, understudies have their own tribe of supporters: a Web site called BroadwayUnderstudies.com is “dedicated to the performers whose names appear on the understudy board and the fans who pray for little white slips of paper to be thrust into their Playbills.” The site—and its Facebook pages and Twitter account—offers updates (“Adam Monley will play Javert in @LesMizBway till end of January”) and posts shout-outs to the possibly overlooked on a big night. (“Happy opening to the understudies of @FiddlerBroadway!”) Jon Rua’s début was noted there, too, accompanied by the hashtag #breakaleg.

It may be asking too much of most ordinary Broadway attendees—who now pay more than a hundred dollars, on average, for a ticket—to be quite so enthusiastic at the unscheduled appearance of a largely unknown or untested performer. But understudies, alternates, stand-ins, and swings deserve to be appreciated—not just for being, as BroadwayUnderstudies.com claims, with partisan overstatement, “Broadway’s hardest working actors” but for the ways in which their unexpected appearances underline what makes live theatre distinctive and, at its best, so thrilling. The presence of an understudy makes vividly clear the fact that a work of art is being freshly created each night. And no matter how practiced or experienced the actor on stage might be, his or her performance, like that of an understudy, has to be summoned and shaped anew, on the spot, under the spotlight.

To see an alternate or understudy in a role can be revelatory—not just about the actor in question but about the work itself. When, in the opening number of “Hamilton,” Lin-Manuel Miranda makes his first entrance, and his character announces himself by name, the audience reliably applauds. Here is the man they have come to see: the first Secretary of the Treasury and the MacArthur-certified genius who has re-imagined him so powerfully. The entrance is an opportunity for a celebration of his achievement. The audience members are fully aware of all that Miranda and Hamilton have accomplished—even though they haven’t yet seen the play, and even though the Hamilton onstage is newly arrived in New York City, unheralded and as yet unknown.

Enacted by an alternate or understudy, though, uncertainty is restored to the scene. When an unfamiliar actor steps into the light as Hamilton, the applause that greets him is scattered, more muted, as audience members rustle their Playbills for the overlooked white slip. The challenge that faces Hamilton, the character, as the play unfolds—how to make an impression against all expectations—is also the challenge that the actor confronts in playing him.

“Who is this kid? What’s he gonna do?” Hamilton’s fellow-rebels ask, before witnessing him deliver his first, incendiary number, “My Shot,” in which he stakes his claim to the future. To see an actor other than Miranda step into this role is to be aware in a different way of the risk and the danger and the promise of the historical moment, and the theatrical one. It is also a salutary reminder to all theatregoers to embrace the understudy, in whatever show he or she appears, in whatever role he or she is cast. See the understudy take his shot. It might be revolutionary.