We show that lepton-pair production in virtual Compton scattering offers, through interference with the well-known Bethe-Heitler process, a sensitive probe to separate longitudinal and transverse response at positive photon invariant masses. This interference can be measured directly in terms of an asymmetry. The role of off-shell effects in the NNγ vertices is investigated as well. An additional NNγγ contact term in the amplitude, included to ensure gauge invariance of the model, cancels a substantial part of the off-shell effects, but introduces ambiguities which affect observables. The response of nucleon resonances is also investigated.