  Motivated by applications to the study of self-force effects in scalar-tensor
theories of gravity, we calculate the self-force exerted on a scalar charge in
a circular orbit about a Reissner-Nordstr\"{o}m black hole. We obtain the
self-force via a mode-sum calculation, and find that our results differ from
recent post-Newtonian calculations even in the slow-motion regime. We compute
the radiative fluxes towards infinity and down the black hole, and verify that
they are balanced by energy dissipated through the local self-force - in
contrast to the reported post-Newtonian results. The self-force and radiative
fluxes depend solely on the black hole's charge-to-mass ratio, the controlling
parameter of the Reissner-Nordstr\"{o}m geometry. They both monotonically
decrease as the black hole reaches extremality. With respect to an extremality
parameter $\epsilon$, the energy flux through the event horizon is found to
scale as $\sim \epsilon^{5/4}$ as $\epsilon \rightarrow 0$.
