In general, the macroscopic plastic deformation — the irreversible part of a material’s total strain — results from rearrangements within its microstructure. In granular materials, the nature of these rearrangements remains difficult to define. To better understand the underlying processes, we developed an experimental method to track the microscopic deformation of a granular packing during loading (in an inclined or vibrated configuration). This approach combines two complementary measurements: correlation of multiply scattered ultrasonic signals (Diffusive Acoustic Wave Spectroscopy, DAWS) and optical tracking of grain displacements. Our observations during loading– unloading cycles reveal the presence of a memory effect (distinct from elasticity) and a second effect related to the degree of reversibility of strain fluctuations. The combination of these two effects gives rise to a robust relationship between the mean plastic strain and its local fluctuations, which is independent of both the frequency and the type of mechanical loading. Finally, when the granular medium is inclined up to avalanche onset, we show that the number and intensity of precursory failure events are strongly influenced by the geometry of the packing and its compatibility with the applied load. Our results make it possible to assess the active or passive role of the precursor regarding to the avalanche.