Real-time measurements of fluctuations arising from electrohydrodynamic instabilities at polymer liquid/air interfaces were used to characterize the selection of a dominant wavelength and the rate at which this fluctuation grows. Reflection interference fringes from the film surface were used to measure the growth rates of fluctuations and their dependence on electrostatic pressure. The growth rate was found to exponentially depend on time and strongly depend on the applied voltage and initial distance between the liquid surface and the opposing electrode. Results at early times are in quantitative agreement with predictions from a linear stability analysis, yet deviations are seen with increasing time and define the limits over which the linearized theory is applicable.