Is it possible to improvise over this kind of music?
In a show I would not be afraid to back a song in this style, but that is with the safety net of improvising with the singer. Having eye contact and listening to their phrasing means that it is possible to give and take a little, end sections at the same time, and pick mood changes.
With just a cold backing track, how could a singer move with the rapid tempo changes, the contrasting emotions? It would be amazing to pull it off, and we would be intruiged to hear your attempts.
If you have the time and the technology (a smart phone would do), we would love you to record and send your results to us at firstname.lastname@example.org. We could include some examples on a future podcst.
We have talked about Sondheim in our musicals podcast if you want to find out a little bit more.
Have fun with it, and do let us know how you got on, even if you dont want to send us your recordings!