  We calculate the free-fall energy density of scalar fields semi-classically
by employing the trace anomaly on a two-dimensional Schwarzschild black hole
with respect to various black hole states in order to clarify whether something
special at the horizon happens or not. For the Boulware state, the energy
density at the horizon is always negative divergent, which is independent of
initial free-fall positions. However, in the Unruh state the initial free-fall
position is responsible for the energy density at the horizon and there is a
critical point to determine the sign of the energy density at the horizon. In
particular, a huge negative energy density appears when the freely falling
observer is dropped just near the horizon. For the Hartle-Hawking state, it may
also be positive or negative depending on the initial free-fall position, but
it is always finite. Finally, we discuss physical consequences of these
calculations.
