In material extrusion additive manufacturing, solid polymer filaments are fed into extruders to generate extrudates needed to build 3D plastic parts. There is a finite gap between the inner surface of an extruder and the edge of a filament, which facilitates the insertion of the filament inside the extruder. It is still not clear how this gap is filled during the extrusion. On the other hand, it is important to know this information, for example, to model the distributions of temperature and pressure during the extrusion. In this work, the gap-filling processes of Acrylonitrile Butadiene Styrene (ABS) and Polylactic Acid (PLA) filaments were directly observed through a heated glass tube, with the aid of particles previously added in the filaments. For both materials, the gap filling inside an extruder was found to be affected by curved profile of the filament, conical geometry of the nozzle and temperature of the extruder.