  The evolution of the black hole horizon can be effectively captured by a
fictitious membrane fluid living on the stretched horizon. We show that the
dynamics of this boundary matter arises from the invariance of the bulk action
under local symmetries in the presence of the inner boundary. If general
covariance is broken in a semi-classical treatment of a quantum field near a
black hole horizon, we argue that it can be restored by the inclusion of a
quantum flux into the membrane conservation equation which is exactly equal to
the Hawking flux.
