We develop a generalized model for studying second-order parametric interactions in 1-D multilayered photonic structures, accounting for collinear oblique waves and partial pump depletion. This model is used to assess the performance of parametric devices in photonic-crystal microcavity (PCM) structures. Our model shows dramatic enhancement of nonlinear interactions at frequencies for which the waves are localized. Also, we demonstrate the exponential dependence of the conversion efficiency of second harmonic generation (SHG) on the number of layers as was recently pointed out. In addition, in optical parametric amplification (OPA), we find that the gain has a resonance-like dependence on the pump intensity, turning large peak gain into strong attenuation at greater intensities, which suggests that the device can operate as an optical switch.