Rotation<IndexTerm> is one of the fundamental parameters that governs the physical structure and evolution of stars. Massive stars are those presenting the highest rotation<IndexTerm> velocities and thus those for which the consequences of rotation<IndexTerm> are the strongest. On the stellar photosphere fast-rotation induces (1) a geometrical flattening and (2) a non-uniform distribution of flux/effective temperature (gravity darkening<IndexTerm> <Secondary>darkening</Secondary> effect). A detailed mapping of these effects on the stellar photosphere, including large scale surface velocity fields, is nowadays possible thanks to modern techniques of optical/infrared long-baseline interferometry<IndexTerm> (OLBI). In this paper we focus on the measurement of gravity darkening<IndexTerm> <Secondary>darkening</Secondary> from OLBI, while the determination of flattening is detailed by Kervella (this volume). In addition, we also show that, for fast-rotators, the combination of OLBI and spectroscopy (spectro-interferometry) allows to go beyond the spatial resolution limit of interferometers in order to measure angular sizes of stars, otherwise not measurable by classical OLBI techniques. The results presented here are based on ESO-VLTI<IndexTerm> interferometric observations of the Be star Achernar.