Measles virus (MeV) is a highly contagious, re-emerging, major human pathogen. Replication requires a viral RNA-dependent RNA polymerase (RdRP) consisting of the large (L) polymerase protein complexed with the homo-tetrameric phosphoprotein (P). In addition, P mediates interaction with the nucleoprotein (N)-encapsidated viral RNA genome. The nature of the P:L interface and RdRP negotiation of the ribonucleoprotein template are poorly understood. Based on biochemical interface mapping, swapping of the central P tetramerization domain (OD) for yeast GCN4, and functional assays, we demonstrate that the MeV P-to-L interface is bipartite, comprising a coiled-coil microdomain proximal to the OD and an unoccupied face of the triangular prism-shaped C-terminal P X-domain (P-XD), which is distinct from the known P-XD face that binds N-tail. Mixed null-mutant P tetramers regained L-binding competence in a ratio-dependent manner and fully reclaimed bioactivity in minireplicon assays and recombinant MeV, demonstrating that the individual L-binding interface elements are physically and mechanistically distinct. P-XD binding competence to L and N was likewise trans-complementable, which, combined with mathematical modeling, enabled the mechanistic characterization of P through two- and stoichiometrically-controlled three-way complementations. Only one each of the four XDs per P tetramer must be L or N binding-competent for bioactivity, but interaction of the same P-XD with L and N was mutually exclusive, and L binding superseded engaging N. Mixed P tetramers with a single, designated L binding-competent P-XD caused significant RdRP hyperactivity, outlining a model of iterative resolution and reformation of the P-XD:L interface regulating polymerase mobility.