  Stars twinkle because their light propagates through the atmosphere. The same
phenomenon is expected on a longer time scale when the light of remote stars
crosses an interstellar turbulent molecular cloud, but it has never been
observed at optical wavelengths. The aim of the study described in this paper
is to fully simulate the scintillation process, starting from the molecular
cloud description as a fractal object, ending with the simulations of
fluctuating stellar light curves. Fast Fourier transforms are first used to
simulate fractal clouds. Then, the illumination pattern resulting from the
crossing of background star light through these refractive clouds is calculated
from a Fresnel integral that also uses fast Fourier transform techniques.
Regularisation procedure and computing limitations are discussed, along with
the effect of spatial and temporal coherency (source size and wavelength
passband). We quantify the expected modulation index of stellar light curves as
a function of the turbulence strength --characterised by the diffraction radius
$R_{diff}$-- and the projected source size, introduce the timing aspects, and
establish connections between the light curve observables and the refractive
cloud. We extend our discussion to clouds with different structure functions
from Kolmogorov-type turbulence. Our study confirms that current telescopes of
~4m with fast-readout, wide-field detectors have the capability of discovering
the first interstellar optical scintillation effects. We also show that this
effect should be unambiguously distinguished from any other type of variability
through the observation of desynchronised light curves, simultaneously measured
by two distant telescopes.
