Before a show, I come very close to puking from nerves and adrenaline. I get up on stage, look at the crowd, and feel like running out the door. On the first note of the first song, all that nervousness and adrenaline flys out of me and cements me into the music, and the crowd keeps me in that place. My brain shuts down and I'm going on instinct. If even one thought enters my head such as what chord I need to play, or what part of the song I need to go to, I lose my grip and screw up. Its a tenuous hold at best, and I don't perform at my best unless I am literally living in the music.
The studio environment is lifeless in comparison to the stage, and I personally find it much harder get into the music. The nerves are different. You feel like you are in front of a judging panel, even though noone may be judging you. There is less excitement in the room, less drive, and nowhere to hide. I find it way more difficult to empty my brain and just go for it. Did I pay that bill? I have to go grocery shopping. I haven't called my mom in awhile. Life creeps in much more easily and pulls you out of the moment.
You mentioned that the vocalist liked working with the cans so that she could hear her mistakes and pitchiness. In my opinion, if you are analyzing your voice while you are recording it then your not fuly committed to the music, and then the performance will suffer. All the technical stuff is for rehearsals IMO, and needs to be forgotten when you step in front of a mic. Its all head games really. Why do a lot of singers have giant egos? Defense. Lets be honest, singers are typically under the most scrutiny. Just rambling here.
