Mechanical testing in situ scanning electron microscopy (SEM) has become a current technique for multiscale micromechanical investigation of polycrystalline materials, because direct observation of deformation microstructures allows identification of strain heterogeneities and related mechanisms. Yet, most of the studies are based on the inherited post mortem microstructures, thus precluding to unravel the local loading history, to understand the development of localization patterns, the potential interactions of concomitant mechanisms and to quantify their respective contributions to the overall strain. We therefore developed a novel experimental setup for thermomechanical testing in situ SEM, especially suited for full strain field measurements based on digital image correlation (DIC) from the sample scale, to the scales of the aggregate and the single grain. We present results obtained during simple compression, at controlled displacement rate and at temperatures up to 400°C, for polycrystalline aluminum presenting randomly oriented coarse grains (ca. 300 m in size). According to the different scales of interest, specific surface marking patterns were realized by electron microlithography. Kinematic analysis by digital image correlation (DIC) allowed to retrieve full surface strain fields. The latter evidenced that the overall viscoplastic response was dominated by crystal slip plasticity. Increasing temperature favored the activation of non-octahedral slip, but also substantial and continuous contribution of grain boundary sliding (GBS). We suggest the latter mechanism as necessary to accommodate local plastic incompatibilities between neighboring grains.