While using additives such as ionic liquids (IL) is known to boost the efficiency and stability of perovskite solar cells, it is still unclear how ILs impact the difficile perovskite film formation process. Here we investigate the impact of the IL BMIMBF₄ on the film formation of the model halide perovskite MAPbI₃ by multimodal optical in situ spectroscopy during solution processing via one-step spin coating and a solvent engineering approach. One-step processing experiments reveal that IL in the precursor solutions does not affect the formation of perovskite–solvent complexes, but higher IL contents delay the perovskite transformation with decreased growth rates. For solvent engineering, the perovskite growth rate decreases with later anti-solvent (AS) dripping as the properties of PbI₄²⁻ species in the precursor solutions change during drying. Here the IL also affects the evolution of the PbI₄²⁻ properties, as the IL cation interacts with the PbI₄²⁻. This interaction appears to reduce the perovskite growth rates after initiating perovskite formation by AS dripping. Still, in the as-coated films the IL efficiently passivates defect states. Thus, our work provides important insights into how decisive ILs impact the sensitive interconnection between precursor properties, film formation process and final optoelectronic functionality of perovskite thin films.