We explored polarization mechanisms at the interface between a dielectric material (an electrolyte) and an insulating liquid, during electrowetting actuation. Native surface charge density due to hydrophobic coating has been measured as an offset voltage for which the contact angle is at its minimum. Surface charge densities as low as 0.023 mC m-2 have been measured using this method, demonstrating that electrowetting can be used as a probe to measure native surface charge density. This effect strongly differs depending on the kind of polarization and is at the origin of major discrepancies between alternative and direct polarization during electrowetting actuation. A new model describing electrowetting actuation is also proposed, leading to a more predictive description as well as useful recommendations on materials to obtain a stable actuation under DC polarization.