It is widely accepted that photoluminescence (PL) properties of colloidal nanocrystals, specifically PbS, are mainly determined by their composition, size, and shape. In this work, we demonstrated that in densely packed PbS nanocrystal films, low-temperature donor-to-acceptor energy transfer is mainly responsible for the PL spectra narrowing and shift toward longer wavelengths. At elevated temperatures, back energy transfer seems to be responsible for an unusual PL intensity temperature dependence and spectra modifications. A reduction of the PbS nanocrystal density, i.e., an increase of a distance separating the nanocrystals, dramatically reduces the effect, and the PL spectra are entirely governed by the nanocrystal size and size distribution.