Sulfide solid electrolytes (SEs) show promise for Li metal solid-state batteries due to their high ionic conductivities and relative ease of manufacturing. However, many sulfide SEs suffer from limited electrochemical stability against Li metal electrodes. In this work, we use a suite ofoperandoanalytical techniques to investigate the dynamics of solid electrolyte interphase (SEI) formation and the associated effects on Li plating. We contrast a sulfide SE that forms an electrically insulating SEI (Li₆PS₅Cl) with an SE that forms an SEI with electrically conducting phases present (Li₁₀GeP₂S₁₂). Using anode-free cell configurations, where the Li/SE interface is formed against a current collector, we perform complimentaryoperandovideo microscopy andoperandoX-ray photoelectron spectroscopy (XPS) experiments. The combination of these techniques allows for the interpretation of electrochemical voltage traces during Li plating. The electrically insulating nature of the SEI in Li₆PS₅Cl facilitates Li metal nucleation and plating after the initial SEI formation. In contrast, in cells that form an electronically conducting SEI, the onset of Li plating is suppressed, which is attributed to a low Faradaic efficiency from continuous SE decomposition. The insights in this study reveal how interphase dynamics control the transition from SEI formation to plating in anode-free solid-state batteries.