Charge and momentum electron densities provide complementary views of cohesive forces in solids. This is particularly true for molecular crystals. The examples of cubic tetracyanoethylene (1,1,2,2-ethenetetracarbonitrile) and its alkali-metal insertion compounds are analyzed from a theoretical point of view. Besides the usual deformation density maps and anisotropy of Compton profiles, it is shown that interaction charge density and interaction Compton profiles can be defined and reveal the subtleties of the intermolecular interactions. It is shown that owing to the large cavities in the crystal, alkali-metal atoms can be inserted, leading to a strong charge transfer to the molecules and to a metallic character; the mechanism of insertion is revealed well by the combination of charge and momentum density studies. The combination of the two techniques of X-ray diffraction and Compton scattering should be of great help in the study of rather weak interactions present in molecular solids.