  The quantum action (dynamical) principle is exploited to investigate the
nature and origin of the Faddeev-Popov (FP) factor in gauge theories without
recourse to path integrals. Gauge invariant as well as gauge non-invariant
interactions are considered to show that the FP factor needs to be modified in
more general cases and expressions for these modifications are derived. In
particular we show that a gauge invariant theory does not necessarily imply the
familiar FP factor for proper quantization.
