I recently ran into a problem with cast thumbnails not loading correctly within my TV Shows and I posted the question -> Weirdness with TV Casts Thumbnails with what I believe is the problem and the solution I used to workaround the issue.
I believe the issue is that when you have Cast thumbnails pulled from TMDB on movies and then if you use TVDB for TV Shows then they conflict with each other causing the TV ones to become blank.
What would be good to have is the ability within the library level (or even agent level) set the specific Casts source. We can do this within the Movie agents but we cannot within the TV Shows.
