[STEPHEN SONDHEIM, spoken]
Sometimes a song changes its shape and texture drastically as you write it. This happened in Follies. Follies is a show about a reunion of chorus girls on the stage of their old theater. It's a party, and one of the girls is reminiscing with another one's husband about their younger days. While doing so, they are shadowed by their younger selves, which are sort of ghosts that trail them through the evening. 

I wrote it as a duet and played it for Jerry Robbins and he said, "You're wasting your resources." He said, "You should utilize both couples, and you should utilize their younger selves as well." So what started as a duet became an octet.