  Stellar activity causes difficulties in the characterization of transiting
exoplanets. Studies have been performed to quantify its impact on infrared
interferometry, but not in the visible domain, which however allows reaching
better angular resolution and is also the one mostly used for spectroscopic and
photometric measurements.
  We use a standard case to completely analyse the impact of an exoplanet and a
spot on interferometric observables and relate it to current instrument
capabilities, taking into account realistic achievable precisions.
  We built a numerical code called COMETS using analytical formulae to perform
a simple comparison of exoplanet and spot signals. We explore instrumental
specificities needed to detect them, like the baseline length required, the
accuracy and S/N. We also discuss the impact of exoplanet and spot parameters
on squared visibility and phase.
  We find that the main improvement to bring is the sensitivity of instruments.
The accuracy on squared visibilities has to be improved by a factor 10 to
detect an exoplanet of 0.1 mas, leading to $<0.5%$ precision, along with phase
measurements of ~$5^{\deg}$ accuracy beyond the first null of visibility. For a
0.05 mas exoplanet, accuracies of ~$0.1%$ and ~$1^{\deg}$ from the first null
are required on squared visibilities and phases, respectively. Magnetic spots
can mimic these signals, leading to false exoplanet characterization. Phases
measurements from the 3rd lobe is needed to distinguish between a spot and an
exoplanet if they have the same radius.
  Increasing interferometer sensitivity, more objects will become common
between interferometric targets and photometric ones. Furthermore, new missions
like PLATO, CHEOPS or TESS will provide bright exoplanet host stars.
Measurements will thus overlap and provide a better characterization of stellar
activity and exoplanet.
