Actuators that bend slice lips are commonly used in cross-directional (CD) control systems for paper making and plastic film extrusion. A dynamic model of these actuators is developed, which is used to investigate their spatial modes and dynamic responses. The model shows that both the shape and the amplitude of the spatial response depend upon the stiffness of the slice lip. The design of most practical CD control systems is simplified by assuming that the spatial and dynamic responses are separable in the sense that each of the spatial modes has the same dynamics. The validity of this assumption is examined and it is shown that subject to conditions that are likely to be satisfied in practice, the responses can be taken as separable. The model is verified using data from plastic film extrusion line and the consequences of separability for the design of CD control systems are discussed.