Mayim Bialik did not actually sing on camera. Bette Midler wanted someone who sounded like she did at that age, so they used someone else's voice.
Lainie Kazan didn't want to play CC's mother, but eventually said yes in order to work with Bette Midler.
Barbara Hershey had collagen lip injections to make herself look younger (she was 40 at the time and was playing Hilary from college age to mid-thirties).
Mayim Bialik and Marcie Leeds reunited to spoof a recreation of the scene where CC and Hilary meet for the first time. The clip of this spoof is featured on Bialik's Facebook page.
The Barbara Hershey screen test scenes were filmed on the Plaza Hotel lobby set of Big Business (1988). The scene where Hillary and C.C. fight at the store, around the middle of the movie, is filmed on the same set (notice the carpet).