Part of the success of The Big Bang Theory is due to its fidelity to itself. It pays attention to its characters, and their interactions, and even their pasts, and adjusts accordingly. Penny (Kaley Cuoco) began the series as an airheaded neighbor, a bubbly blonde object of unattainable desire. But she quickly deepened as a character, showing sides of herself as both a frustrated wannabe actress and a sarcastic have-to-work waitress. Her on-again, off-again mating dance with neighbor Leonard (Johnny Galecki) took years, literally, to come to fruition – and recently, Penny’s past was revisited when a former boyfriend, Zach (Brian Thomas Smith), made an untimely reappearance, representing the sort of hunky stupid guys she used to date, as opposed to her current brainy scientist husband. Tonight, because the Big Bang writers always seem to know good things when they stumble on them, Zach returns again – this time with a job offer for Penny that may be the type of offer she can’t refuse.