The growth of Ruddlesden–Popper perovskite thin films of organic lead halides is complicated by the existence of multiple crystallization pathways available to precursors in solution. During thin-film growth processes, such as spin-coating or blade-coating, solvents can evaporate too quickly to clearly resolve different reaction intermediates and products that form during crystallization. Here, we resolve multiple reaction products and intermediates that form during growth of (C4H9NH3)2(CH3NH3)n−1PbnI3n+1 Ruddlesden–Popper compounds by studying drop-cast precursor solutions through the evolution of X-ray diffraction, photoluminescence, and optical micrographs in situ over long timescales in a thin-film geometry. We found that methylammonium-rich solvate intermediates play a crucial role in directing the bulk optical properties of the films and form simultaneously with smaller regions of Ruddlesden–Popper phases during growth. The microstructure and optical properties of these sub-phases were characterized during growth and after annealing, revealing that discrepancies between thin-film and single-crystal optical properties originate from solvate intermediates. These lower-band-gap minority phases dominate the optical emission spectrum by means of rapid energy migration and contribute to sub-band-gap electronic states in photovoltaic devices. Processing routes to yield thin films with optical properties similar to single crystals of Ruddlesden–Popper phases were developed by tuning the precursor stoichiometry and deposition kinetics.