Three-dimensional X-ray diffuse scattering data have been collected at room temperature for form (II) of the trimorphic molecular system p-(N-methylbenzylidene)-p-methylaniline. Although this polymorph has been reported to have a perfectly normal ordered average structure, strong and highly structured diffuse scattering was observed, indicating that substantial thermal disorder is present. A diffuse scattering analysis has been carried out using Monte Carlo simulation techniques. Narrow streaks of intensity extending between Bragg peaks in the h0l section were found to arise from planes of diffuse scattering in three dimensions. These are caused by highly correlated molecular displacements along chains of end-to-end disposed molecules running in the a-c direction, corresponding to methyl-methyl intermolecular interactions. A second significant feature--rods of diffuse scattering running in the b* direction--indicates that molecular layers normal to b have a tendency to undergo lateral displacements with little correlation between layers. Finally, the internal flexibility of the molecule is required for a best fit. Changes in the two dihedral angles are found to be strongly correlated and show large excursions (> +/- 20 degrees) from the average values. All of these features suggest possible mechanisms for the way in which form (II) might transform to other polymorphs.