Anode-free lithium metal batteries (ALMBs) are promising high-energy storage systems, but their practical operation is limited by unstable Li nucleation, non-uniform deposition, and continuous interfacial degradation on Cu current collectors. Here, we investigate MgF2 as a reactive precursor for component-resolved interfacial regulation rather than as a passive lithiophilic coating. Comparative analysis of bare Cu, Mg-coated Cu, and MgF2-coated Cu reveals that pristine MgF2 exhibits limited intrinsic affinity toward Li, whereas its beneficial function emerges after electrochemical activation. During initial lithiation, MgF2 undergoes conversion to form a functional interphase in which Mg-associated species promote Li nucleation and F-derived species contribute to a LiF-rich solid electrolyte interphase. Thickness-dependent measurements further show that an optimized MgF2 precursor layer is required to balance precursor supply and conversion kinetics. The activated MgF2-derived interphase lowers the nucleation barrier, stabilizes Li plating/stripping, and promotes dense, uniform Li deposition, as confirmed by ex-situ electron microscopy, synchrotron X-ray computed tomography, and X-ray photoelectron spectroscopy. These findings provide mechanistic guidelines for designing conversion-type precursor layers for stable ALMBs.