So Robert Downey Jr. showed up earlier this week at the end of “Castle” to do this little banter thing with “Castle” star Nathan Fillion, the former leading man of Joss Whedon’s “Firefly”. Whedon, of course, directed “The Avengers”, which RDJ stars in as Iron Man. Also, in the week’s episode of “Castle”, Fillion’s “Firefly” co-star Adam Baldwin guest-stars. Head — exploding!!!!









