  We compare the effects of timelike, lightlike and spacelike one-dimensional
inhomogeneities on the probability of nonperturbative pair production in strong
fields. Using interpolating coordinates we give a unifying picture in which the
effect of the inhomogeneity is encoded in branch cuts and poles circulated by
complex worldline instantons. For spacelike inhomogeneities the length of the
cut is related to the existence of critical points, while for lightlike
inhomogeneities the cut contracts to a pole and the instantons become
contractable to points, leading to simplifications particular to the lightlike
case. We calculate the effective action in fields with up to three nonzero
components, and investigate its behaviour under changes in field dependence.
