While other numerical and analytic techniques exist, the modal expansion is one of the few that provides physical insight into the anatomy of the Casimir Effect. Although it is very simple in a small number of situations, this kind of analysis becomes quite involved when phenomena such as dissipation in the media or complicated geometries are taken into account for the calculation of the Casimir effect. Moreover, in this case, its connection and equivalence to other approaches becomes blurry and puzzling. The knowledge of the interplay between (quantum) thermodynamics, geometry, dissipation and mode decomposition can be important to understand the strength, the sign and, in general, the behavior of the Casimir force. Here, we investigate some aspects of the modal approach to Casimir forces, also presenting experimentally relevant examples.