  Studying the behaviour of a quantum field in a classical, curved, spacetime
is an extraordinary task which nobody is able to take on at present time.
Independently by the fact that such problem is not likely to be solved soon,
still we possess the instruments to perform exact predictions in special,
highly symmetric, conditions. Aim of the present contribution is to show how it
is possible to extract quantitative information about a variety of physical
phenomena in very general situations by virtue of the so-called Hamilton-Jacobi
method. In particular, we shall prove the agreement of such semi-classical
method with exact results of quantum field theoretic calculations.
