The real beauty of this puzzle lies in the superb grid construction: a complete thematic set incorporated with elegant symmetry, proper words throughout, everything just the right length. Good clues added the icing. I had only one minor reservation, which was that I would have expected the encoded instruction to have resulted in more extensive work than it did.
The theme emerges clearly if you write the correct letters beside the rows and under the columns, when only one letter appears to be out of place (but isn't, of course). It wouldn't have been too difficult to rectify that.
I knew the theme from having seen my daughter perform in a production of Stephen Sondheim's relevant work at the Edinburgh Festival a few years ago. It's not often performed, but worth catching when it comes round.