This is from John's blog
Here's some interesting history on the sound of the Four Seasons and some behind-the-scenes info on how JERSEY BOYS re-creates that sound:
The signature sound of the Four Seasons, originally called "The Four Seasons, featuring the sound of Frankie Valli" was created by having Valli record his vocal part twice and then running them back together. If you listen to the original recording of "Sherry" you will hear TWO Frankie Vallis! This technique was called "doubling," and the Four Seasons were one of its earliest pioneers, using it on some of their most famous songs.
When we recorded the JERSEY BOYS cast album, we were able to do exactly the same thing in the studio. If you listen to the JERSEY BOYS CD carefully you'll hear TWO John Lloyd Youngs on some of the signature up-tempo Seasons songs (for instance, track #5, "Sherry"). Other vocal parts are doubled, as well, just as in any choral arrangement.
To recreate that sound live, onstage, JERSEY BOYS uses live offstage or onstage singers just like in the chorus of any other musical, (except a doubler sings in unison with the lead vocal line, not in harmony). Doubling is a way to layer support under a sound, not a replacement for it, which is why you'd hear it if I made a vocal mistake: there's no place to hide. You can always hear me singing every single note every time I open my mouth; the doubling is there to help recreate that full studio Seasons sound, NEVER to replace my voice or sing for me. Doubling is not used on any of my ballads or solo numbers during the show; it's only those signature four-part harmonies (which, in reality, can have as many as ten voices onstage and off supporting each of the four vocal parts). It happens in parts of the show where I'm not one of the singers: look for other characters onstage singing in the early scenes of JERSEY BOYS where the "Variety Trio" is performing -- those are doublers.
Duct tape is like the force. It has a light side, a dark side, and it holds the world together."