Complete conductivity spectra extending from de to mid-IR frequencies have been taken of crystalline AgCl, AgBr and beta-AgI, at various temperatures. The spectra contain two new dynamic features : (i) a thermally activated Debye-type relaxation which is explained by the frequent hopping of silver ions from their regular lattice sites to adjacent interstitial sites and back again and (ii) contributions to the conductivity below and around 1 THz which are attributed to high-amplitude vibrational motions, mostly of the silver ions. We also report results of a neutron scattering study on AgBr. These contain thermally activated quasielastic components caused by the localized cation back-and-forth hopping and fast correlated movements of neighboring ions.