Bernadette Peter's has the status as being the highest profile voice actor on Animaniacs and which is why for any episode in which she appears the credits state "and Bernadette Peters as Rita". The only exception was for the season 4 episode 8 segment "Mindy in Wonderland" when she voiced "Cheshire Cat".
According to composer Steven Bernstein, the animation of the piano concerto was done before any music was composed for it, so he had to match the music to what the pianist was doing in the animation.