  High-$T_c$ superconductors are usually described as strongly correlated
electronic systems. This feature deeply affects the one-particle and
two-particle properties of the system. In particular, a large incoherent
background developes on the top of a narrow quasi-particle peak in the
one-electron spectral function. We schematize this structure with a simple
phenomenological form. The corresponding Green's function is employed to
calculate the charge response of the system taking into account in a proper way
strong correlation effects. The effective charge interaction acquires a
structure in the exchanged momentum space with a predominance of forward
scattering, in agreement with previous numerical calculations. The consequences
of the momentum dependence of the interaction are discussed in the framework of
the nonadiabatic theory of superconductivity proposed for the high-$T_c$
materials.
