  We introduce an operator version of the BRST-BFV effective action for
arbitrary systems with first-class constraints. Using the Schwinger action
principle we calculate the propagators corresponding to: (i) the parametrized
non-relativistic free particle, (ii) the relativistic free particle and (iii)
the spining relativistic free particle. Our calculation correctly imposes the
BRST-invariance at the end-points. The precise use of the additional boundary
terms required in the description of fermionic variables is also incorporated.
