  A large fraction of red-supergiant stars seem to be enshrouded by
circumstellar material (CSM) at the time of explosion. Relative to explosions
in a vacuum, this CSM causes both a luminosity boost at early times as well as
the presence of symmetric emission lines with a narrow core and
electron-scattering wings typical of type IIn supernovae (SNe). For this study,
we performed radiation-hydrodynamics and radiative transfer calculations for a
variety of CSM configurations (i.e., compact, extended, and detached) and
documented the resulting ejecta and radiation properties. We find that models
with a dense, compact, and massive CSM of ~0.5Msun can match the early
luminosity boost of type II-P SNe but fail to produce type IIn-like spectral
signatures (aka ``flash features''). These only arise if the photon mean free
path in the CSM is large enough (i.e, if the density is low enough) to allow
for a radiative precursor through a long-lived (i.e., a day to a week),
radially extended unshocked optically thick CSM. The greater radiative losses
and kinetic-energy extraction in this case boost the luminosity even for modest
CSM masses -- this boost is delayed for a detached CSM. The inadequate
assumption of high CSM density, in which the shock travels quasi adiabatically,
overestimates the CSM mass and associated mass-loss rate. Our simulations also
indicate that type IIn-like spectral signatures last as long as there is
optically-thick unshocked CSM. Constraining the CSM structure therefore
requires a combination of light curves and spectra, rather than photometry
alone. We emphasize that for a given total energy, the radiation excess
fostered by the presence of CSM comes at the expense of kinetic energy, as
evidenced by the disappearance of the fastest ejecta material and the
accumulation of mass in a dense shell. Both effects can be constrained from
spectra well after the interaction phase.
