The performer must connect emotionally with the song--be it humor, anger, irony, or sorrow, or whatever. Then it is up to the performer to be the song's instrument. The performer must let the song "sing" and not allow his/her own emoting to get in the way. It is a fine line to follow--to allow the mood of the song to shine without giving in to histrionics. Sometimes we get it right, sometimes we don't, but when it's exactly right--whew-ee!!!

Here's a related question--we all have our own ideas of some great songs. What is the ineffable atmosphere that causes the same song to be the high point of some performances and the kiss of death at others? Is this a separate thread?