In this chapter we detail various experimental approaches to characterize the fuzziness of complexes made of the C-terminal domain of the nucleoprotein (NTAIL) from three representative paramyxoviruses and of the C-terminal X domain (XD) of the homologous phosphoprotein. We discuss the advantages, the limitations, as well as the caveats of the various methods. We describe experimental data showing that paramyxoviral NTAIL-XD complexes are characterized by a considerable amount of conformational heterogeneity. We also detail recent data that revealed that NTAIL is highly malleable, i.e., it displays a partner-mediated polymorphism. All the results suggest that NTAIL plasticity and fuzziness play a role in the coordination and regulation of the NTAIL interaction network so as to ensure efficient transcription and replication.