The antisolvent dripping time during spin-coating of CH3NH3PbI3 (MAPbI3) strongly impacts film morphology as well as possible formation of the intermediate precursor phase, and – consequently – device performance. Here, we use in situ photoluminescence (PL) to directly monitor the fast-occurring changes during MAPbI3 synthesis. These measurements reveal how the ideal timing of the antisolvent leads to homogeneous nucleation and pinhole-free films. In addition, these films show significantly reduced nonradiative recombination with 1.5 orders of magnitude difference in absolute PL quantum yield compared to films where no antisolvent is applied. Low-temperature PL measurements confirm that antisolvent treatment reduces the number of trap states presumably in the bulk material. However, if the antisolvent is dripped late, heterogeneous nucleation via the orthorhombic (MA)2(DMF)2Pb3I8 intermediate phase leads to a needle-like morphology that can be correlated to a red-shifted in situ PL signature. We find that the ideal dripping window is very narrow when using dimethylformamide as the solvent, confirmed by device performance metrics. Finally, the use of in situ PL is discussed to gain information on nucleation, growth and ultimately increase reproducibility.