I'm no actor, so any back story I come up with for the characters (just how long were Gertrude and Claudius an item? where is Cordelia's mom?) is for my own amusement. If you are an actor, then the back story is obviously part of who you (at least temporarily) are.
So, tell us one. Tell us the most interesting or unexpected back story you've ever come up with for a character. What came first, the text or the idea? Did you imagine a back story and then find supporting evidence in the text to work off of? Or vice versa, did you get a brainstorm after reading something in the text, and expanded that backward?
It's a Friday afternoon and I don't get my best traffic on Fridays so I don't know how many actors we'll get to chime in, but I'm hoping to see a couple of different backstory interpretations of the same character. I think that could be enlightening.