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Rent-heads prepare to be jealous: actor Marc de la Cruz has dated both Idina Menzel and Anthony Rapp. Okay, it was onstage in If/Then, but that kind of opportunity only happens to a special breed of Broadway actors: the "swing."

All theatregoers are familiar with an understudy: that name on the white slip in Playbill; the one that gives you a pang of disappointment — and, sometimes, unexpected delight. But there are various subsets of understudies.

At If/Then, there is the "stand-by," whose sole function is to replace a specific character; like Jackie Burns, who went on for Idina Menzel (for the first time) in the middle of a performance.

Then there's the "cover," an ensemble actor who steps into a principal role, as Ann Sanders did when playing Anne, the love interest of Kate (LaChanze). "My husband said he went to kiss me the next morning and I looked disappointed," Sanders reports. "When he asked why, I said, 'Your lips aren't nearly as soft as LaChanze's. Hers are like butter.'" Then there's the "swing," the offstage actor who steps in to fill both the ensemble parts and the principals. Swings have their own "put-in" rehearsals, but the first time they get to perform with the actual cast and sing with the orchestra onstage is in front of a paying audience. Since ensemble actors often play multiple roles during a performance, their parts are referred to as "tracks." Marc de la Cruz covers six tracks in If/Then, each with its own separate staging and harmony lines. "The offstage choreography is just as important as the onstage choreography," de la Cruz says. "You have to know what the person does once they exit or how soon they can get to their place for their next entrance." Blinded by sidelighting, one wrong turn could send a swing careening into another actor or a piece of scenery.

But both cast and crew guide a replacement through the performance, a practice called "shove with love."

"The very first time I went on there was a moment onstage when Idina made a point to make eye contact with me and squeeze my hand," de la Cruz says. "It was such a simple gesture but it immediately made me feel more at ease. She had to have known how nervous I was; it was my Broadway debut!"

De la Cruz describes that night as "surreal." Swings have "the rare privilege of experiencing the show from both sides," he explains, often watching the show from the audience for weeks. "In the opening number, Idina was singing downstage center and it suddenly struck me that this was the same thing I'd seen every night except this time I was behind her instead of in front."

"That's how Sting must feel," he adds, referring to the rock legend now performing in The Last Ship.

Over at Beautiful: The Carole King Musical, swing Sara Sheperd also describes the experience as "surreal." As the back-up to the cover for leading lady Jessie Mueller, Sheperd spent over a year at the ready, periodically going on in three other roles. Then, in one ten-day period, she went on at every performance, not only as Carole, but also Carole's mother, Carole's best friend, and the woman who sleeps with Carole's husband. Beautiful's tracking system is tight, which is why someone just a few years out of college can end up playing a woman twice her age. Shepard agrees with director Marc Bruni's assertion that "Hopefully the wig and the costume go a long way to make it as believable as possible."

Unlike If/Then, where actors of different races cover one another (a distinction that won its casting director, Telsey & Co., an Actors' Equity Award for "Extraordinary Excellence in Diversity on Broadway"), the races of the real-life pop groups the Shirelles and The Drifters aren't flexible. "You can't have a white Shirelle or a white Drifter," Bruni says. "Early drafts of the show had the Monkees and the Animals," he adds, "but the cast size would have gotten out of control."

One male swing, Melvin Tunstall, covers all four of the Drifters. "It's a bit of a gamble," Bruni admits, which is why any Drifter who takes vacation is covered specifically by a special "vacation swing," leaving the show's one swing available to cover the others. "We want to hedge against a three Drifters show," Bruni says. "That is a non-option." Still, back-up plans frequently fail. Actor-musician Brandon Ellis learned that lesson the hard way — twice — when he signed on to cover five tracks in Once, playing 15 instruments. Ellis was instructed to focus on three particular tracks first, but another actor's unexpected trip to the emergency room forced Ellis to go on for one of those two other tracks — with no rehearsal. "It was a complete Actor's Nightmare," Ellis says.

Then Ellis was given two weeks notice to go on for the other neglected track. That would have been luxurious had the part not required Ellis to play the piano, an instrument he didn't know. "I taught myself from an iPhone app," he says, "then played in public for the first time on national television — on a piano that was moving around the stage."

So the next time one of those little white slips falls out of your Playbill, pay special attention to see if a swing is going on. They're among Broadway's most hard-working actors.