MIDSEASON RETURN: Rose McIver returns as Ivy, in an episode that displays just how much fun she and executive producer Rob Thomas are having with this series. The premise is that Ivy, though infected by a zombie, staves off the effects of full-out zombie-dom thanks to a steady diet of brains from the local coroner – her place of employment. A side effect of her brain diet is that she temporarily absorbs the memories, emotions and skills of the people she “samples,” which makes each week a chameleonic acting exercise for McIver. She’s terrific at it, and tonight, the person whose brain she tastes, and whose memories and attitudes she adopts, is that of a murdered librarian – a seemingly meek woman who, in turns out, had a secret life as an author of erotic stories. The episode title is “Fifty Shades of Grey Matter,” and in addition to seeing what McIver does with this personality adjustment, we get to hear voiceovers from the point of view of the late author – played by Kristen Bell, the leading lady of Thomas’ previous prime-time series, Veronica Mars.