How To Write A Massive Cast Of Characters

So today I want to talk about dealing with a massive cast of character.  I am talking about main and secondary characters (not the odd village baker passing through a random scene never to be heard from again). If you're not sure what I mean by a large cast of characters, think Lord of the Rings.  As well as the main cast of the Fellowship, there were also additional characters that came in and hung around for a while.  They were fleshed out and with their own thread in the plot.