Rick Rubin Vocal Quotations

He sang differently when he was playing the guitar, ... There was a purity to the vocal that was more natural and less of a performance. He was less able to think about what he was singing and it sounded better.

I noticed that when he played guitar, he was a lot different singer than the guy I saw on the big stage. It was less of this big performance. There was more feeling. It's probably as simple as you can't think about how you phrase the vocal if you are also concentrating on guitar.