In the study of strongly interacting systems, correlations on the two-particle level are receiving more and more attention. In this work, we study a particular two-particle correlation function: the fermion-boson vertex. It describes the response of the Green's function when an external field is applied and is an important ingredient of diagrammatic extensions of dynamical mean-field theory. We provide several perspectives on this object, using Ward identities, sum rules, perturbative analysis, and asymptotic relations. We then use these tools to study the vertex across the doping-driven metal-insulator transition and find a divergent imaginary part.