  We discuss a general scheme for a construction of linear conformally
invariant differential operators from curved Casimir operators; we then
explicitly carry this out for several examples. Apart from demonstrating the
efficacy of the approach via curved Casimirs, this shows that this method
applies both in regular and in singular infinitesimal character, and also that
it can be used to construct standard as well as non--standard operators. The
examples treated include conformally invariant operators with leading term, in
one case, a square of the Laplacian, and in another case, a cube of the
Laplacian.
