  We simplify and generalize an approach proposed by Di Vecchia and Ravndal to
describe a massive Dirac particle in external vector and scalar fields. Two
different path integral representations for the propagator are derived
systematically without the usual five-dimensional extension and shown to be
equivalent due to the supersymmetry of the action. They correspond to a
projection on the mass of the particle either continuously or at the end of the
time evolution. It is shown that the supersymmetry transformations are
generated by shifting and scaling the supertimes and the invariant difference
of two supertimes is given for the general case. A nonrelativistic reduction of
the relativistic propagator leads to a three-dimensional path integral with the
usual Pauli Hamiltonian. By integrating out the photons we obtain the effective
action for quenched QED and use it to derive the gauge-transformation
properties of the general Green function of the theory.
