  We discuss the effects from the Kaluza-Klein modes in the brane world
scenario when an interaction between bulk and brane fields is included. We
focus on the bulk inflaton model, where a bulk field $\Psi$ drives inflation in
an almost $AdS_5$ bulk bounded by an inflating brane. We couple $\Psi$ to a
brane scalar field $\phi$ representing matter on the brane. The bulk field
$\Psi$ is assumed to have a light mode, whose mass depends on the expectation
value of $\phi$. To estimate the effects from the KK modes, we compute the
1-loop effective potential $V_\eff(\phi)$. With no tuning of the parameters of
the model, the vacuum becomes (meta)stable -- $V_\eff(\phi)$ develops a true
vacuum at a nonzero $\phi$. In the true vacuum, the light mode of $\Psi$
becomes heavy, degenerates with the KK modes and decays. We comment on some
implications for the bulk inflaton model. Also, we clarify some aspects of the
renormalization procedure in the thin wall approximation, and show that the
fluctuations in the bulk and on the brane are closely related.
