The electrodynamics of a nonlinear optical film sandwiched between two dielectrics has been analyzed for two possible switching modes. One corresponds to the case where the incident laser is initially internally reflected (total internal reflection, TIR) and the film has a positive nonlinearity. The second is for the case where the laser is initially transmitted, but the optical film has a negative nonlinearity. Two factors that have hitherto been largely neglected have been taken into account. One factor is the intense (near unity) reflection from the two interfaces enclosing the optical thin film, which, coupled to the nonlinearity of the film, gives rise to nonlinear Fabry-Perot action. The second factor is the time the evanescent field takes (in the TIR case) to create the required refractive index change in the film for transmission to occur. Several of the analytic results have been experimentally verified using nematic liquid crystal film as the nonlinear medium.