  Light interaction with optical cavities is of fundamental interest to enhance
the light-matter interaction and to shape the spectral features of the
electromagnetic fields. Important efforts have been carried out to develop
modal theories of open optical cavities relying on an expansion of the fields
on the eigen-fields of the cavity. Here, we show how such an expansion predicts
the temporal dynamics of optical resonators. We consider a Fabry-Perot cavity
to derive the full analytical expressions of the internal and scattered field
on the quasi-normal modes basis together with the complex eigen-frequencies. We
evince the convergence and accuracy of this expansion before deriving the
impulse response function (IRF) of the open cavity. We benefit from this modal
expansion and IRF to demonstrate that the eigen-modes of the open cavity impact
the signals only during the transient regimes and not in the permanent regime.
