  We apply the light-front reduction of the Bethe-Salpeter equation to matrix
elements of the electromagnetic current between bound states. Using a simple
(1+1)-dimensional model to calculate form factors, we focus on two cases. In
one case, the interaction is dominated by a term instantaneous in light-cone
time. Here effects of higher Fock states are negligible and the form factor can
be effectively expressed using non-wave function vertices and crossed
interactions. If the interaction is not instantaneous, non-wave function
vertices are replaced by contributions from higher Fock states. These higher
Fock components arise from the covariant formalism via the energy poles of the
Bethe-Salpeter vertex and the electromagnetic vertex. The replacement of
non-wave function vertices in time-ordered perturbation theory is a theorem
which directly extends to generalized parton distributions, e.g., in (3+1)
dimensions.
