An actuator assembly for operating a setting member includes an actuator body undergoing a dimensional change upon applying electrical energy thereto; a preliminary pressure chamber for accommodating liquid; a diaphragm sealing the preliminary pressure chamber and being displaceable by the actuator body; a valve member carried by the diaphragm; a transfer chamber for accommodating liquid; and a displaceable plate body disposed in the preliminary pressure chamber and sealingly bounding the transfer chamber. The diaphragm, as it is moved by the actuator body, displaces the plate body. A displaceable piston body sealingly bounds the transfer chamber and has a surface which is smaller than the surface of the plate body, whereby the plate body--with the intermediary of the liquid--displaces the piston body to a greater extent than the displacement of the plate body. The piston body, as it moves, displaces the setting member. A valve opening provided in the plate body is closed or opened by the valve member carried by the diaphragm. In the open position of the valve member, the valve opening establishes a hydraulic communication between the preliminary pressure chamber and the transfer chamber.