Faults in the Earth experience a wide range of stress perturbations; hence, understanding the evolution of fault strength in response to stress changes is crucial for unraveling earthquake generation mechanism. Although extensive experimental research has been conducted, a fundamental understanding of the relevant microscopic processes is still lacking. Here, we investigate the behavior of a sheared granular layer under normal stress step tests. Our results reveal that step increases in normal stress induce a two-stage shear stress response: an initial rapid linear increase, followed by a slip-dependent nonelastic evolution phase, after which the system reaches a new stable state. Shear velocity sharply decreases during the linear phase and gradually recovers to the background level during the nonlinear phase. The absolute magnitude of normal stress increase regulates the amount of slip required to reach a steady state. Conversely, the relative size of the normal stress step controls the perturbation in slip rate. At lower shear velocities, step increases in normal stress more readily trigger fault locking. Microscopic analysis shows that these macroscopic shear responses are closely related to changes in the contact area and the population of slipping and frozen contacts. These findings provide new insights into the grain-scale mechanics that govern fault behavior under variable normal stress.