A new method for measuring the width of an interface in the presence of overhangs and holes is presented, using a local Gibbs dividing surface. It is compared with the conventional method based on the order-parameter profile. Dynamics and size dependence of an interface are studied numerically in the two dimensional Q2R cellular automaton. Two types of interface fluctuations can be distinguished: the capillary waves dominate on length scales larger than the bulk correlation length, whereas order parameter fluctuations on small scales give rise to an intrinsic interface width. The scaling behavior of the capillary waves is shown to be the same as for the Ising model. The divergence of the intrinsic width at the critical point and its critical slowing down are investigated.