Willoughby Kipling, who debuts in this episode, is directly based on a character from the comics that bares a shocking resemblance to another DC character: John Constantine. This is because the comic writer initially wanted to use Constantine for the story but was declined (because the story was too weird), so he made a replacement character.
The episode is entirely based on Grant Morrison's run.
Mark Sheppard and Alan Tudyk appeared in the first episode of Firefly (2002) and both also appeared on Dollhouse (2009).
Matt Bomer & Mark Sheppard have been working together in White Collar.