Polymer composites based on poly(lactic acid) (PLA) and three fillers, graphene oxide (GO), silica nanoparticles, and a nanohybrid consisting of silica doped GO, were prepared and investigated. A combination of complimentary techniques, namely thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), polarized light microscopy (PLM), differential scanning calorimetry (DSC), and broadband dielectric spectroscopy (BDS), were employed to study the polymer-filler interaction, crystallization and molecular mobility. For the low filler content of 1 wt%, all inclusions act as nucleating agents, increasing the rate of PLA crystallization. Among all fillers, including the mixture of pristine silica and GO, the nanohybrid imposes the stronger effects on crystallization rate, the latter being highly desirable in industrial processing of PLA. The crystalline fraction (similar to 30%) was found to be the dominant factor in molecular mobility, is agreement with previous investigations on PLA based systems. Based on the degree of polymer/filler interaction (FTIR) and the suppression in calorimetric/dielectric strength of the glass transition (DSC/BDS), the interfacial rigid amorphous fraction (RAF) could be estimated. Interestingly, RAF exhibits its largest value for the composite based on the hybrid filler, in qualitative agreement between the three techniques. The latter fraction, representative of the polymer adsorption onto the nanofillers, is considered important within many applications (thin polymeric films, polymer nanocomposites, etc), furthermore, it is widely believed that RAF dominates the observed improvements of the properties in the nanocomposites.