[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