<p>This study investigates the contribution of inter-ply friction to the dominant deformation mechanisms in multi-ply non-crimp fabric (NCF) layups during the forming of complex three-dimensional geometries. A modified cantilevered bending test was developed to assess the influence of increasing inter-ply friction on the non-linear bending behavior of multi-ply stacks. Furthermore, the effect of inter-ply friction on in-plane shear behavior was examined experimentally under vacuum pressure, simulating the conditions encountered during double diaphragm forming (DDF). The results demonstrate that both bending stiffness and shear resistance of the laminate stack increase with enhanced ply compaction, which generates greater inter-ply frictional forces. A DDF case study involving a 16-ply NCF preform highlights the impact of elevated bending stiffness and shear resistance on formability. Notably, increased frictional forces arising from variations in ply stacking sequences can adversely affect formability, as evidenced by the formation of pronounced out-of-plane wrinkles. This systematic investigation reveals that the final geometry of the demonstrator preform is governed more significantly by in-plane shear resistance than by out-of-plane bending behavior. These findings underscore the critical role of inter-ply friction in shaping the deformation response of multi-ply NCFs and offer insights for optimising preform design and processing strategies in advanced composite manufacturing.</p>