The RNA polymerase (RNApol) of respiratory syncytial virus (RSV), the main agent responsible for bronchiolitis infections, is a viral RNA synthesis machinery composed of at least of 2 proteins, a 250 kDa catalytic subunit and a 4x27 kDa phosphoprotein co-factor (RSV P). RNApol associates with the viral nucleocapsid, a ribonucleic complex containing the genomic viral RNA matrix, as well as with several other proteins, either viral co-factors or cellular enzymes and signaling proteins. Low affinity towards its partners ensures processivity of the polymerase. Notably many proteins, involved in the RSV RNApol and its complexes display low structural complexity regions, RSV P in particular, which is a linchpin of these complexes. We designed a combination of biochemical and NMR approaches to gain structural and dynamic insight into this protein, which is arranged as a tetramer, but otherwise lacks a defined 3-dimensional structure. NMR revealed structural features ranging from fully disordered regions to ordered regions in the form of isolated helices. This is an advantage for a protein that acts as a platform to recruit specific protein partners either to the RNApol or to replication sites in infected cells. RSV P contains transient secondary structures, which provide plasticity of P and act as contact regions that lead to compaction of the protein or are recognized by protein partners. They may be stabilized in protein complexes. We identified at least four regions that correspond to different partners and complexes. One of these is the RSV M2-1 protein that keeps the RNApol in transcription mode. We also characterized the PP1 phosphatase binding site on P, in the vicinity of that of its substrate, M2-1. Finally, we used NMR for probing protein-protein interaction inhibitors targeting the binding of the nucleocapsid to P, which could pave the way for new antiviral therapeutics.