When I first moved to Nashville many years ago, I was singing backgrounds in a "simul-session". These sessions were where the musicians, background vocalists and lead singers recorded together at the same time. Like live TV- if you flubbed it was extremely obvious to everyone!
Anyway, this session was for none other than Johnny Cash. As we prepared to record, I remember that I watched a studio tech take the strings off his guitar and give it to him to hold while he was singing. His wise producer had noticed he sang better when attached to his guitar! At the time I wondered what that was all about; now I understand.
When a singer is accustomed to performing well with something in the hands (be it guitar, piano or just mic), there is a subtle balance adjustment in the body memorized by the muscles. When you take the instrument or mic out of the hands for studio singing, the singer usually drops hands to their sides. Big mistake, because the lifeless arms and hands usually become "rib anchors", crushing in the ribcage and interfering with breath control.
You don't want to mess with your breath control. All kinds of odd things start happening -- to sum it up, you just don't feel comfortable singing, and you can hear that in playback. Your pitch, tone, stamina and style "lics" suffer. You become nervous, lose confidence, assume a more guarded posture and everything gets worse.
It's important to get this terrible chain of events going the other way. Learn to use your hands and arms in ways that mimic the playing of your instrument. I recommend putting your fingertips together to cause the ribcage to stay wide. If you need to put a dummy mic in your hand to synthesize the feeling, do it! Get help from a coach who can show you how to do these things BEFORE you go into the studio.
Singing for Johnny Cash was truly an honor. I'm glad I moved to Nashville early enough to get in on this and all the other great historic sessions. I learned so much from my mentors and teachers, and I'm happy to pass it on. My thanks to Hurshel Wiginton of the legendary background vocal group the Nashville Edition for hiring me.
Do you have any instances of having to sing without your usual gear? How did you do?