A systematic atomic-scale analysis is presented of the interactions of SiH radicals from a silane plasma with amorphous hydrogenated silicon (a-Si:H) film surfaces based on molecular dynamics and Monte Carlo simulations. The coordination of the silicon surface atoms and the hydrogen surface concentration determine the mechanisms by which the SiH radical interacts with the surface and attaches itself to the film, as well as the "sticking" probability of the radical. On a-Si:H surfaces with low hydrogen concentration, SiH radicals remain on the surface and attach to silicon dangling bonds. In contrast, on a-Si:H surfaces with high hydrogen concentration and lower density of dangling bonds, radicals penetrate into the bulk film and attach to subsurface silicon atoms. An average reaction probability of 95% is predicted, which is in excellent agreement with recent experimental data.