"The tempo, the way the characters talk, their voices, what they said was almost lyrical," Stallone has said of the impetus. "I always thought this would work."
Disenchantment with 1990's Rocky V drove Stallone to seriously contemplate it for the first time.
"I wasn't happy with how that one turned out," he said. "I thought maybe [a musical] is how [ Rocky] should have its finale. Everyone thought it was absurd."
A decade ago, Stallone began actively lobbying for a stage version, requesting a meeting with Thomas Meehan, who had won Tony Awards for writing the books to the musicals Annie, The Producers, and Hairspray.
"I said, ' Rocky? That's crazy! There's no way I'm going to do that,' " Meehan recalled. "Then I met with him at his home in Beverly Hills. We watched the movie together, and I thought, 'He may be right.' "
"There's a terrific Cinderella story of a scorned guy and an unattractive girl. And then there's a David and Goliath story about a guy people think is a bum getting a shot at redemption. It's a simple but very strong, emotional story."
Rocky maintains many of the details from the original film, and the introduction of objects like the raw eggs for Rocky's training breakfast or the sides of beef hanging in Paulie's meat warehouse have been drawing enthusiastic whoops of recognition from preview audiences.
The score includes Bill Conti's "Gonna Fly Now" theme as well as "Eye of the Tiger" (which was actually written for 1982's Rocky III). There are also more than a dozen new songs from composer Stephen Flaherty and lyricist Lynn Ahrens.
The two big numbers for Rocky (Andy Karl) - "Fight From the Heart" and "Keep on Standing" - sound distinctly like Bon Jovi power ballads.
The play goes to great lengths to root itself in its 1975 Philadelphia setting. Rocky's radio alarm wakes him to Wibbage. There are numerous drop-ins of WPVI news bulletins, with period aquamarine graphics from Channel 6. A news conference is held at the Rittenhouse Hotel, and the climactic fight takes place at the Spectrum.
(There are tone-deaf notes as well: In one big production number, Rocky is hailed as a "Southside Celebrity." In Chicago maybe.)
Rocky lavished enormous time and resources on two elements: getting the cast's Philadelphia accents right and mounting the final 20-minute championship bout between Rocky and Apollo Creed (Terence Archie).
Before rehearsals started, dialect coach Kate Wilson gave everyone a detailed glossary of South Philly pronunciations. "Are Dennis loves the winner" = "Our dentist loves the winter." "Wall Women bridtch" = "Walt Whitman Bridge."
She also provided the cast with a Dropbox folder - three hours of videos of native Philadelphians talking. "I wanted the cast to see it as well as hear it," Wilson said. "Actors usually just go with a generic Brooklyn accent, but they sound different in Philly. They even move their hands differently."
Fortunately, the two leads, Karl and Margo Seibert (Adrian), didn't need much coaching. They're both from the Baltimore area.
"They were 90 percent of the way there," Wilson said. "A lot of Philadelphia's melodies and vowel shifts come right up from Baltimore. We hear some of the wonderful lilt and a bit of the swoop in their rhythms."
But Wilson wanted to take it a step further, to ensure that Karl's Rocky sounded slightly different talking to Paulie from when he was addressing Apollo.
"Philly natives really code shift," she said. "They can turn it on at a Phillies game and then turn it way down if they're talking to someone from New York."
After all that preparation, Karl on stage ends up sounding more like New York comedian Ray Romano than he does faux-Philadelphian Stallone.
But the guy can take a punch, which is the most important attribute in playing Rocky. Karl proves his chin in the electrifying battle with the unbeaten champion.
It's a showstopper. Literally. The ticket holders in the first eight rows are escorted from their seats up on to the stage to sit in the "Spectrum bleachers." The boxing ring then slides out into audience, and a big Diamondvision screen descends from the rafters.
Then the heavyweights go toe to toe.
"I'm dedicating a portion of every performance to GNC and my trainer at the gym," Karl joked via e-mail.
"It's amazing how caught up people get in the fight," said Meehan, who attended every preview. "They react like they're watching a real fight."
Some take it even further.
"There was a group of older women the other night," recalled associate fight coordinator Patrick McCollum, "and during the fight, they were all raising their fists up in front of their faces and shifting from side to side. I doubt they ever fought a day in their lives, but they were right in the ring with Rocky and Apollo."
Maybe that's why Rocky, for better or worse, has become Philadelphia's most recognizable and enduring cinema icon and why he stands a puncher's chance on Broadway. The story of a man who wants to go the distance so he won't be considered just another bum from the neighborhood is so reliably inspiring.
Even Karl has gotten caught up in the spirit. A few months ago, he paid a pilgrimage to the Art Museum steps.
"I ran into Mike [Kunda], the self-proclaimed 'best Rocky impersonator,' taking pictures by the Rocky Statue," Karl said. "He started talking about the fact that he auditioned for that new Rocky musical. He had no idea who I was or that I had been hired to play Rocky.
"Anyway, Mike was going on and on about the fact that he 'couldn't get in to audition' and that 'they hired some guy who doesn't even look like Rocky.'
"I was just smiling and laughing and enjoying it for what it was. Mike finally caught on that I might be involved with the show. Then he treated me like his best friend in the world. It was pretty hilarious. And he only charged me $5 to have my picture taken with him."
Yo, Rock, you got off cheap. It's $59 and up if you want he should sing.
"Rocky on Broadway" at the Winter Garden Theatre, 1634 Broadway (between West 50th and 51st Streets), New York. Information: www.rockybroadway.com or 212-239-6200.