I heard Archie Fisher discussing this with Dave Burland. There are some songs which
provoke such a strong emotional response in the singer that, in order to perform them without fear of being overcome in the process, he/she has to "play them in" (in the privacy of their own homes). Now, we can all imagine the sad ones which do this, but are there any too funny, too embarrassing, too angry etc? I think I would struggle to get through "The Bantam Cock" by J Thackery, for example, in front of a maiden aunt.