The spatial region that is in the vicinity of an interface and actually affects the interface response, and hence the reflected wavefield, is of particular interest for the characterization of reflectors from a seismic viewpoint. This region is represented by a volume of integration of medium properties above and below the interface whose maximum lateral extent corresponds to the lateral extent of the interface Fresnel zone, and whose maximum vertical extent is equal to a thickness we evaluate approximately for subcritical incidence angles for a plane interface as well as for curved interfaces of anticline and syncline type. The maximum vertical extent might be larger than the seismic wavelengths for subcritical incidence angles close to the critical angle and for a strong impedance contrast at the interface. Although the part of the reflector volume lying below the interface and affecting traveltime measurements actually is smaller than described in previous studies, the whole part of the reflector volume that affects the amplitude of the reflected wavefield is larger than estimates in previous studies, which considered only the spatial region below the interface. For a syncline (respectively, an anticline), it is larger (respectively, smaller) than described for a plane interface. In addition to providing more physical insights into the wave reflection process, this study might have significant implications for seismic interpretation using amplitude-variation-with-angle methodologies.