Three-dimensional modeling of the solid motion in a lab-scale rotating drum has been conducted via the discrete element method. After validating the simulated results with available experimental data, the active-passive interface was identified, following which particle-scale information in these two regions, in particular the influences of fill level and rotating velocity, were obtained. The results demonstrate that: (1) the total number of particles in the passive region is three times that in the active, (2) the transverse and axial velocities span a wider range in the active region, with the transverse values being greater, (3) the collision force is much higher in the active region, with the greatest magnitudes in the y direction relative to that in the x and z directions, (4) particle displacements are generally lower and have a narrower distribution in the active region, (5) the local solid residence time (SRT) distribution profiles are similar axially in that the highest SRT magnitudes are at the center region of the bed, while the other parts of the bed have uniform SRT magnitudes. (c) 2016 American Institute of Chemical Engineers AIChE J, 62: 3874-3888, 2016