The peculiar role of the substrate field on the nonlinear optical response of a molecular adsorbate experiencing a resonant Raman scattering process is analyzed. The description of the internal dynamics is done in terms of the renormalized normal mode coordinates that account for the conformational changes induced in the adsorbate by the substrate field. This analytical description, well-adapted to weak substrate fields, points out the underlying mechanisms that are responsible for the enhancement of the resonant Raman signal observed in some molecules. They involve elementary processes bringing additional intermediate vibrational states into play, depending on the nature of the field inhomogeneities. Also, according to the relative values of the unperturbed equilibrium positions of the modes, their displacements, and frequency changes due to the substrate field, enhancement as well as decrease of the signal can possibly be observed and are evaluated. Of course, for a strong substrate field, the number of intermediate vibrational levels participating in the Raman process becomes very large, and a complete numerical evaluation is required.