AVClub: Not every single episode of a sitcom is going to move its characters forward in a way that makes an impact, either emotionally or in terms of the overall narrative. In fact, most sitcoms only wave at the idea of character progression from one episode to the next. Typically, there are two or three “event” episodes in a year where something big happens, and that’s meant to make up for all the previous stagnation. Think of how Howard and Bernadette’s relationship was dead in the water early this season, up until it was revealed Bernadette was pregnant in one of those “event” episodes. Still, it’s rewarding when sitcoms manage to sneak in low-key character moments in episodes that are otherwise mostly reserved for comedy. “The Line Substitution Solution” is one of those episodes.