A hypothesis for fiber-fiber interaction in planar randomly oriented concentrated fiber suspensions is proposed and tested. The idea is that at sufficiently high fiber concentrations, friction and lubrication at fiber-fiber contact points are the dominant interaction mechanisms. A fiber pull-out technique is introduced to measure the force per unit fiber length on a single longitudinally moving fiber embedded in a volume of bulk suspension. By varying both the fiber velocity and the fiber volume fraction, the lubrication and frictional components of the force are identified. Furthermore, the corresponding bulk shear viscosity resulting from the same mechanisms is derived and compared with experimental data. The results support the hypothesis.