We're not casting directors, so we can't begin to imagine how challenging of a job it must be. That being said, there really are days when it seems near-impossible to imagine how they could have thought certain casting decisions were a good idea. It happens, and sometimes the results can be a tremendous surprise in a good way.
There have also been some train wrecks, of course. It's hard sometimes to tell whether it's the acting, the script, bad direction or a lethal combo of all of them. What are some of the casting choices that you consider the most misguided?