  We define a set of boundary conditions that ensure the presence of a null
hypersurface with the essential characteristics of a horizon, using the
formalism of weakly isolated horizons as a guide. We then determine the
diffeomorphisms that preserve these boundary conditions, and derive a dynamical
action for these diffeomorphisms in a neighbourhood of the horizon. The action
is similar to that of Liouville theory, and the equation of motion of the
gravitational degrees of freedom approaches that of a free two-dimensional
conformal field theory in the near-horizon region.
