Understanding how a frictional interface fails, and how past ruptures will affect the propagation of the following ruptures is fundamental to a better comprehension of earthquake behavior. In that sense, numerical simulations provide a useful tool to study the dynamics of slip. In this study, we model a setup similar to laboratory friction experiments [Rubinstein et al., 2007 ; Ben-David et al. 2010] using the finite-element method. We model a 2D block of viscoelastic material in contact with a rigid body. A slip-weakening friction law controls the friction at the interface. Special care is taken to apply appropriate regularization and viscosity in the simulation. Our model allows to investigate the onset and evolution of sliding at a frictional interface. The block is loaded by applying shear loading on the trailing edge, which results in a high concentration of shear stresses close to that edge, where slip will initiate. The simulated slip events are consistent with experimental observations [Rubinstein et al., 2007]: we observe a sequence of slip precursors, which initiate at shear levels well below the global static friction threshold. These precursors stop before propagating over the entire interface, and their length increase with increasing shear force. Such precursors to global sliding significantly modify interface stresses, and affect the onset and propagation of global sliding. We analyze in details the changes affecting interface stresses. In particular, we evaluate how the stress concentration created by the arrest of one precursor event evolves when the following events take place. We show that each slip event does not renew the interface, but that stress heterogeneities are maintained after several slip events. Each slip event thus propagates in a highly heterogeneous stress field, which leads to important variations in the propagation velocity of the slip front.