  Broadband optical and narrowband Si XIII X-ray images of the young Galactic
supernova remnant Cas A obtained over several decades are used to investigate
spatial and temporal correlations on both large and small scales. The data
consist of optical and near infrared ground-based and Hubble Space Telescope
images taken between 1951 and 2011, and X-ray images from Einstein, ROSAT, and
Chandra taken between 1979 and 2013. We find weak spatial correlations between
the remnant's emission features on large scales, but several cases of good
optical/X-ray correlations on small scales for features which have brightened
due to recent interaction with the reverse shock. We also find instances where:
(i) a time delay is observed between the appearance of a feature's optical and
X-ray emissions, (ii) displacements of several arcseconds between a feature's
X-ray and optical emission peaks and, (iii) regions showing no corresponding
X-ray or optical emissions. To explain this behavior, we propose a
inhomogeneous model for Cas A's ejecta consisting of small, dense optically
emitting knots (n ~ 10^(2-3)/cm^(3)) and a much lower density (n ~ 0.1 -
1/cm^(3)) diffuse X-ray emitting component often spatially associated with
optical emission knots. The X-ray emitting component is sometimes linked to
optical clumps through shock induced mass ablation generating trailing material
leading to spatially offset X-ray/optical emissions. A range of ejecta
densities can also explain the observed X-ray/optical time delays since the
remnant's 5000 km/s reverse shock heats dense ejecta clumps to temperatures
around 3x10^4 K relatively quickly which then become optically bright while
more diffuse ejecta become X-ray bright on longer timescales. Highly
inhomogeneous ejecta as proposed here for Cas A may help explain some of the
X-ray/opticalfeatures seen in other young core collapse SN remnants.
