The central engine of AGN is too compact to be observed directly so its structure has to be inferred through the combination of spectral and variability information. Here we study the connection between optical-emitting accretion disc and X-ray emitting corona of the Seyfert NGC3783. Short light curves are consistent with optical fluctuations arising from X-ray reprocessing, but the long-term fluctuations found over the course of several years prove that at least long-term optical variability is not produced by the X-rays: at time-scales longer than 1 year the variability power in the optical is higher than in the X-rays. This indicates variability originating in the thin accretion disc itself, therefore capable of revealing its characteristics.