A nonequilibrium molecular dynamics algorithm is employed to study a fluid confined between two immobile walls. The linear response theory of Bocquet and Barrat [Phys. Rev. E 49, 3079 (1994)] is used to relate phenomenological parameters, which determine the boundary conditions of the interface, to averages over microscopic quantities that can be obtained directly from the simulation. The parameters, which characterize "stick" or "slip" behavior and the thickness of the immobile fluid layer at the boundaries, have been determined for a model system : namely, a soft-sphere fluid interacting with corrugated walls.