Nathan Fillion knows that a “man’s gotta do what a man’s gotta do,” to quote his now infamous Captain Hammer from Dr. Horrible’s Sing-Along Blog.  And on his new show, Castle, what a man (in this case, Richard Castle, best-selling author) has to do (besides be adorably charming) is follow around a pretty homicide detective (Kate Beckett, played by Stana Katic) in an effort to help her find the serial killer behind the life-imitates-art murder spree that brings to life