It happened one amazing December day in 1956: Elvis Presley, Johnny Cash, Jerry Lee Lewis and Carl Perkins played an impromptu jam session at Sun Records in Memphis.

It was an informal outing; the songs sung, but not in full, by four guys in street clothes early in their careers, their trajectories not entirely certain. Still, the music and conversations were recorded, including Lewis’s religious rant about rock n’ roll.

Decades later, the session inspired Million Dollar Quartet, the new musical set to open on Broadway following a killer run in Chicago.

“The first time I saw the photo of them at the session, I said, ‘Are you kidding? These guys were together?’” says Lance Guest, who plays Cash in the show.

Written by Colin Escott and Floyd Mutrux and directed by Eric Schaeffer, the show required no new music since the legendary gathering netted about two dozen vintage songs, from “Great Balls of Fire” and “Folsom Prison Blues” to “Down by the Riverside” and “Sixteen Tons.”

The musical takes place entirely in a recreated Sun studio owned by seminal early rock n’ roll figure Sam Phillips, played by Tony nominee Hunter Foster.

On the day the quartet came together, Elvis (Eddie Clendening) was already riding the fame train thanks to his hit, “Heartbreak Hotel,” while Cash, Perkins (Rob Lyons) and Lewis (Levi Kreis) were trailing the King in their breakthroughs.

“Sam Phillips was weighing the possibility of RCA buying him out and Elvis had been famous for just a year-and-a-half,” Guest says. “Carl had just had a hit with ‘Blue Suede Shoes’ under his own name, but when Elvis sang it on TV, everybody thought it was an Elvis song; Johnny had done ‘I Walk the Line,’ and Jerry Lee was new. Each one of them had something at stake, except Jerry Lee who just wanted people to notice him.”

Guest says that with about an hour of Million Dollar Quartet devoted to music, there is only 40 minutes or so to tell the stories of the four artists and Phillips.

“That’s the challenge, to tell you who these characters are, what they need, what they want, how they interact with each other,” he says, adding that he doesn’t think the show should be categorized as a jukebox musical. “The difference,” he says, “is that we’re not restricted to one artist’s catalogue and we tie the songs together thematically. The tone of the music follows the dramatic arc and the content of their stories.”

Guest, a long-time fan of Cash, received his live At San Quentin concert album as a ninth birthday gift. “I thought ‘A Boy Named Sue’ was hilarious,” he recalls.

The difficult part about portraying Cash, he says, is “to maintain the innocence and purity” Johnny displayed in the mid-’50s, before he established his “Man in Black” persona. “He was a pretty introverted guy who wanted to be a gospel singer at that time in his life,” Guest adds.

As for Clendening, Guest says he “has to show the human Elvis, not the hotshot showman he was,” while Kreis has the most complicated physical stuff, because of Lewis’s pull-out-all-the-stops piano playing. During a performance in Seattle, he punctuated the end of a number by jumping over the piano and tore a ligament in one of his knees.

According to Guest, he was on the ground, with people in the audience wondering if this was part of the show. Still, Kreis was back on stage in character the next day -- albeit not nearly so animated. “He sat down during the show until he was O.K.,” recalls Guest.

As for Lyons, because Perkins is not nearly as well known as the others, he has the greatest artistic license. “There aren’t many Perkins impersonators,” Guest concedes.

Having been together for most of Million Dollar Quartet’s 17-month run in the Windy City, the four musician-actors have a solid stage dynamic, as well as a formidable audience hook.

By the end, Guest says, theatergoers leave the theatre the saying, “I can’t believe you guys did this.”

Million Dollar Quartet begins previews at the Nederlander Theatre (208 W. 41st St.) on March 13th for an April 11th opening. For reservations, call 212-307-4100 or click here.