SINGING passionately from head and chest and heart, with full voice and a distinctively soaring (yet macho) falsetto, has always come naturally to Frankie Valli. So recalls this one-of-a-kind stylist's longtime musical partner and friend for all seasons, Bob Gaudio.

"The man just opened his mouth and that's what came out," Gaudio marveled earlier this week from Nashville, remembering his first "meet" with Valli more than five decades ago (1958) in Newark, N.J. "He's what I call an inside-out singer, a natural. He's a classic Italian street singer, like Frank Sinatra, like Jimmy Roselli. He has a huge amount of heart. And you believe every word he puts forward."

But for all the guys who've taken on the iconic Valli role in "Jersey Boys," the long-running musical newly returned for its third stay at Forrest Theater, the portrayal is fraught with peril, never to be taken on lightly.

In fact, the job's so tough it's almost always shared - in the current Philadelphia production by four different guys, one visible, others working or waiting in the wings. And to make sure they won't flunk out, candidates for the pop star part must first take on and survive a special training program. "Frankie Bootcamp" insiders call it. Think a rigorous singing/dancing/acting program at a Manhattan rehearsal studio that tests not only the would-be's gifts but equally their staying power in a most vocally challenging role.

"We learned this lesson after the first workshop production at the La Jolla [Calif.] Playhouse in late 2004," Gaudio said. "Our first Frankie, David Norona, signed up for six weeks. The show did so well we extended the run for four months. By the end, he'd been ravaged, lost his voice, tragically couldn't move with us to Broadway. We had to recast the part, start all over again with John Lloyd Young, who wound up winning a Tony (2006) and recently completed the Frankie role in Clint Eastwood's movie version due out next June. John actually played the part eight times a week for a year and a half, God bless him, but he was the last who would."

Fit to be Frankie

"The demands for the role are very tough and very specific," said Richard Hester, the "Jersey Boys" production supervisor who got involved "before we even had a script" (essentially retelling Valli/Gaudio's own story) "that evolved the project from a mere 'jukebox musical' into something much more."

To play Valli, Hester detailed, "you have to look Mediterranean, stand 5-foot-9 or less (the real deal is 5'5" or 5'6"), be able to act convincingly and dance extensively, as well as sing up a storm on all those classic Valli solo and Four Seasons songs - "Can't Take My Eyes Off of You," "Sherry," "Walk Like a Man," "Oh What a Night," "Ragdoll" and a mere 15 or so more.

"And just being able to go into a falsetto isn't good enough," added Ron Melrose, the show's music supervisor/arranger who shepherds contenders through the boot camp alongside movement coaches and vocal specialist Katie Agresta, a former opera singer who's toughened many a famous rocker for the rigors of the road.

"Most adult male performers who can still sing up there in the higher octaves do so in a gentle, timid, choir boy voice," Melrose continued in his call from LA, where yet another Valli casting call was underway. "We're searching for the rare ones that are vocal monsters, that can reach into the upper two octaves with a gritty nasality. They can't and shouldn't be exactly Frankie Valli clones, but they have to get close."

Auditions, Joisey style

Ten role-seeking finalists at a time earn admission to the Frankie boot camp - whittled down from an initial casting call that may number as many as 500. ("Why haven't they chronicled this process in a reality TV show?" you might wonder.)

"Maybe four" come out alive, said Hester - after passing final mike-training sessions with Gaudio in Nashville - to be plopped in the waiting pool for a Frankie opening in one of the three U.S. productions (Broadway, Las Vegas and touring) or one of the several playing overseas.

Yes, this Italo-American rags-to-riches story is still pulling crowds in London after six years, in Germany and the Netherlands, recently played in South Africa and Turkey, and is coming soon to South Korea. It floors even the principals that the show's had such sturdy "legs," breaking down language and cultural barriers. " 'Mamma Mia' and 'Smokey Joe's Cafe' were our role models, now 'Jersey Boys' is the inspiration for newer song-catalogue shows,"Gaudio said.

The talent pool of Frankies empties faster than you might suspect, because in any given production there are four trained singers/actors/dancers ready and rarin' to take on the vocally strenuous, perpetual motion part, shared Nick Cosgrove, now the six-times-a-week Frankie fronting the Forrest ensemble.

"There's also a second Frankie who spells me two performances each week. That's how I started, as a two-show Frankie; they like to ease you in. And because stuff can happen we have two more waiting in the wings who can also fill Frankie's shoes during emergencies - one who normally plays the Joe Pesci part, and a fourth, the show's swing player who covers several roles."

In notoriously parched Las Vegas, the occupational hazard of "Vegas throat" is so severe no actor fills the role more than four times a week - on (real) Frankie Valli's orders, Gaudio said.

But when not on stage, all those extra Vallis are often hovering in the wings behind microphones, fleshing out the vocal harmonies that the original Mr. V would overdub himself in the recording studio.

"We work very hard to emulate the original records' multilayered sound," Hester said. "Where other shows might lavish money on costumes and scenery, we spend extra on the audio equipment. And we do it so well showgoers often question whether the music has been prerecorded. It's not!"

The now 25-year-old Nick Cosgrove admits he focused on his own private "Frankie Bootcamp" regimen for a long, long time before winning the role.

"I first saw 'Jersey Boys' in Chicago as a high school senior and thought, 'Wow, I want to do this part someday. I need to train for this.' So I did - at Carnegie Mellon in Pittsburgh. It didn't hurt that I looked right - my mom's 100 percent Italian. And was seriously into this music. My parents only let us kids listen to the oldies channel. 'Can't Take My Eyes Off of You' was the first song I ever sang and luckily I had the range for it. And coming full circle, one of the first places I wound up playing Frankie was at the same Chicago theater where I'd seen 'Jersey Boys.' It's been a dream come true."

Forrest Theater, 1114 Walnut St., $42-$137, through Jan. 5, 215-893-1999, kimmelcenter.org.
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