  We investigate why half of X-ray--selected active galactic nuclei (AGN) in
deep surveys lack signs of accretion in their optical spectra. The majority of
these ``optically--dull'' AGN are no more than 6 times fainter than their host
galaxies in rest-frame R-band; as such, AGN lines are unlikely to be
overwhelmed by stellar continuum in at least half the sample. We find that
optically--dull AGN have the mid--infrared emission and L(x)/L(IR) ratios
characteristic of local Seyferts, suggesting that the cause of optical dullness
is not missing UV--optical continua. We compare the morphologies of 22
optically--dull and 9 optically--active AGN at 0.5<z<0.8, and find that
optically--dull AGN show a wide range of axis ratio, but optically--active AGN
have only very round axis ratios. We conclude that hard X-rays select AGN in
host galaxies with a wide range of inclination angle, but only those AGN in the
most face-on or spheroidal host galaxies show optical emission lines. Thus,
extranuclear dust in the host galaxy plays an important role in hiding the
emission lines of optically--dull AGN.
