She never got to be shown on screen, but Carol Ann Susi, the offscreen voice of Howard’s mother, was a very funny element of this show. The actress’ unexpected death last November left the producers with a sad quandary: How to note the character’s absence? Tonight’s episode does just that, while also making room for guest star Nathan Fillion, playing himself. As in, for these guys, the hero of Firefly, and perhaps even the villain of Dr. Horrible’s Sing-Along Blog. Yeah, he’s also the star of Castle, but this crew values a different type of pop-culture currency.