Fast electrons accelerated by lasers into solids expand into vacuum from the target rear surface. They also transfer their energy to target bulk electrons, inducing target expansion into vacuum. Both the low-density cloud of fast electrons, as well as the expansion gradient of the high-density, cold target have been measured via optical probe reflectometry. This allows accessing the time- and space-resolved dynamics of the fast electron density and temperature and of the bulk (cold) electrons temperature. In particular, indicates that the mean fast electron energy, as seen at the target rear side, is a decreasing function of the target thickness.