With their exceptional optoelectronic properties, CsPbBr₃ perovskite nanocrystals (NCs) are promising materials for light-emitting devices. Elucidating their stimulated emission mechanisms is fundamental to grasp the limitations hindering their use as electrically pumped lasers. In particular, two questions remain open: why the Amplified Spontaneous Emission (ASE) band is significantly shifted from the fluorescence one, and why the former seems to suddenly emerge from, and coexist with, the latter. Here, these questions are addressed through experimental ASE measurements, combined with numerical simulations and a comprehensive assessment of the performance of different analytical expressions used in the literature to retrieve the optical gain from these experiments. This multi-facet study shows that the ASE behavior in CsPbBr₃ NCs thin films stems from four distinctive processes: reabsorption due to a large overlap between the absorption and fluorescence spectra, a strong contribution of excited state absorption within the fluorescence window, the excitation of differently polarized waveguide modes, and the coexistence of short- and long-lived localized excitons. The results in this work delineate the best practices to analyze the optical gain in perovskite samples, help to understand their ASE mechanisms, and provide insights to boost their lasing efficiency.