  An automorphism $f$ of a closed orientable surface $\Sigma$ is said to be
extendable over the 3-sphere $S^3$ if $f$ extends to an automorphism of the
pair $(S^3, \Sigma)$ with respect to some embedding $\Sigma \hookrightarrow
S^3$. We prove that if an automorphism of a genus-2 surface $\Sigma$ is
extendable over $S^3$, then $f$ extends to an automorphism of the pair $(S^3,
\Sigma)$ with respect to an embedding $\Sigma \hookrightarrow S^3$ such that
$\Sigma$ bounds genus-2 handlebodies on both sides. The classification of
essential annuli in the exterior of genus-2 handlebodies embedded in $S^3$ due
to Ozawa and the second author plays a key role.
