During an earthquake, fault slip weakening is often explained by frictional heating phenomena, generally promoting melt production on the fault surface. Here, we investigate the influence of melt production at the scale of the grains composing a fault gouge. We use a modern version of Discrete Element Modelling (DEM) able to deal with realistic grain shapes, and couple it with a Multibody Meshfree Approach able to provide a satisfactory proxy for the mechanical behaviour of molten grains. Frictional sliding of solid grains and viscous shearing of molten grains are monitored during simulations. Our results confirm the natural tendency of granular gouge to localize deformation in a thin layer, and thus to trigger local melt production. We also show that the appearance of melt is likely to enhance this localization, and might create a positive feedback to its own production. We propose guidelines for the future writing of a friction model inspired by these simulation results.