  We present the characterization and initial results from the QUEST-La Silla
AGN variability survey. This is an effort to obtain well sampled optical light
curves in extragalactic fields with unique multi-wavelength observations. We
present photometry obtained from 2010 to 2012 in the XMM-COSMOS field, which
was observed over 150 nights using the QUEST camera on the ESO-Schmidt
telescope. The survey uses a broadband filter, the $Q$-band, similar to the
union of the $g$ and the $r$ filters, achieving an intrinsic photometric
dispersion of $0.05$ mag, and a systematic error of $0.05$ mag in the
zero-point. Since some detectors of the camera show significant non-linearity,
we use a linear correlation to fit the zero-points as a function of the
instrumental magnitudes, thus obtaining a good correction to the non-linear
behavior of these detectors. We obtain good photometry to an equivalent
limiting magnitude of $r\sim 20.5$. Studying the optical variability of X-ray
detected sources in the XMM-COSMOS field, we find that the survey is
$\sim75-80$% complete to magnitudes $r\sim20$, and $\sim67$% complete to a
magnitude $r\sim21$. The determination and parameterization of the structure
function (${SF}_{norm}(\tau) = A \tau^{\gamma}$) of the variable sources shows
that most BL AGN are characterized by $A > 0.1$ and $\gamma > 0.025$. It is
further shown that variable NL AGN and GAL sources occupying the same parameter
space in $A$ and $\gamma$ are very likely to correspond to obscured or low
luminosity AGN. Our samples are, however, small, and we expect to revisit these
results using larger samples with longer light curves obtained as part of our
ongoing survey.
