Strong light-matter coupling gives rise to a superposition of light and matter states called polaritons. In the molecular setting, strong light-matter coupling is usually achieved when a large number of molecules is coupled to the same mode of an optical cavity. In this case, the energy structure is normally described as consisting of two distinct polaritonic states together with a large set of intermediate molecular dark states, that due to their purely molecular nature are believed to exhibit dynamics that resemble those of the bare molecular states. In this work we investigate the nature of these intermediate states and how their dynamics and optical response deviate from those of molecules outside the cavity. We apply our framework to simulate linear and non-linear optical responses of an experimentally studied system, namely 4CzIPN, to gain insight into how these intermediate states are involved in and modify intersystem crossing and reverse intersystem crossing under the strong light matter coupling regime.