As a performer, I experience what performers throughout time have experienced: the rehearsals can be tedious, or focus on rote, or focus on getting down specific words, movements or music. But that magic — that “organic” quality that makes it seem as if it has not been carefully rehearsed (often over and over and over) — is what usually emerges in live performance so audiences never know the work that went into it before. Here’s a perfect glimpse of the process.
Watch Judy Garland doing her run through for an ABC 1966 Hollywood Palace variety show (singing one of the more grating songs of that era). Some of it is perfunctory. She forgets the words. The energy is not at full throttle. And then watch her doing it for the actual show (at this point in her career her voice was not at its best and she had a long history of alcohol and drug abuse, which eventually killed her). Notice the difference.

Yes, and the fact that you can discern her dislike of the song is part of her profound truth as a performer.