Amazing that, with all the years I've been involved with the theatre I've never delved into the etymology of this pharse (although I have wondered about it, and discussed it with fellow thespians, but never actually researched it). I will be going on a Google excavation!

I have also done extensive research on the character and life of John Wilkes Booth (whom I sometimes portray at Civil War reenactments), and I have never seen one mention alluding to his breaking his leg at the time of Lincoln's assasination as having given rise to this phrase.

And, by the way...NEVER say "break a leg" to a dancer! I studied tap for many years, so I know.