It's silly, but Daryl Hannah as Morticia? Nobody could replace Caroline Jones, but why not that Lilith actress from Cheers?

Martin Sheen in Apoc. Now.

I think it was Pauline Kael who pointed out that Meryl Streep was making a career of being miscast--she could sing country western note perfect but you don't believe a note of it.

Worst feared--Juliet Binoche made such a counterintuitive but brilliant choice for the woman in The Unbearable Lightness of Being, smiling and laughing as a woman for whom life was "heavy"--worst would be Streep in that role. Or if Nick Nolte had got the part as Han Solo. Robin Williams setting out to prove again that every profession is about being a comedian.