  Nonfactorizable contributions to exclusive two-body nonleptonic weak decays
of heavy mesons arising from color octet currents, characterized by the
parameter $r_2$, are extracted from the data. It is found that $r_2$ is equal
to $\sim -0.67\,$, $-(0.9\sim 1.1),~-(1.2\sim 1.3)$, $\sim 0.36$ respectively
for $D\to \bar{K}\pi,~\bar{K}^*\pi,~\bar{K}^*\rho,~\bar{B}\to D\pi$ decays. As
expected, soft-gluon effects become stronger when the decay particles are less
energetic, allowing more time for significant final-state strong interactions.
As a consequence, the parameter $a_2$ is not universal and is channel or class
dependent, contrary to the anticipation of the factorization approach. The
leading $1/N_c$ expansion works most successfully for $D\to\bar{K}\pi$ decays
as the subleading $1/N_c$ factorizable contribution is almost compensated by
the soft-gluon effect. We argue that, in contrast to what happens in $B^-\to
D^{(*)} \pi(\rho)$ decays, the nonfactorizable term and the subleading $1/N_c$
factorizable term in $\bar{B}\to \psi\bar{K}^{(*)}$ decays are opposite in
signs, in accordance with a recent QCD sum rule calculation. Implications are
discussed.
