It'a a scene from her brilliant story Lines of Descent .
http://www.sugarquill.net/read.php?storyid=1701&chapno=1
If you haven't read this story yet, get yourself there now. This is quite seriously one of the best (and certainly most powerful) HP fanfics I have read.

This scene is from Chapter 3, where Remus and Elizabeth discuss the facts of lycanthropic life.