Viscoplastic properties of polycrystalline materials condition many aspects of our everyday life, as for example, hot forming and durability of metallic structures at high temperature, glacier flow, or plate tectonics powered by convection of Earth’s mantle rocks. In general, it is admitted that viscoplastic deformation of polycrystals is largely dominated by crystal slip plasticity (CSP). Interfacial mechanisms, as grain boundary sliding (GBS) are mostly invoked for superpastic behaviour, favoured at high temperatures, small grain sizes and low strain rates. However, numerous studies evidence that often both mechanisms coexist. Still, very few have focussed on their respective contributions to the global deformation process. Besides, the way these mechanisms interact remains unclear. These questions are the aim of the present work. We have studied the viscoplastic response to uniaxial compression of two different classes of annealed and un-textured polycrystalline CFC materials: ionic NaCl and Aluminium, characterized by coarse and equilibrated polygonal grains (ca. 300 m). Aiming specifically at the localization aspects and mechanisms identification, we realized 2D full strain field micromechanical characterization, based on in situ SEM multi-scale observations and digital image correlation (DIC). Additionally, NaCl samples were analysed by in situ synchrotron X-ray tomography, so that we obtained 3D full strain fields. Our results clearly show that for both materials CSP and GBS co-exist. Besides, their interactions are co-operative: CSP is undoubtedly the dominant strain cumulative mechanism. Though, GBS continuously acts as a secondary (but necessary) mechanism, allowing for accommodation of local grain-to-grain strain incompatibilities, resulting from the inherently anisotropic nature of crystal slip. Both mechanisms are absolutely necessary to ensure macroscopically homogeneous flow. For both materials, we show how a minor (but crucial) contribution of GBS allows the development of localization bands and ductile strain propagation throughout the microstructure