Saturday Night Live began its 50th season last fall, around the same time as the premiere of Jason Reitman’s film Saturday Night, which dramatizes the program’s 1975 debut. All of this has put fans into something of a retrospective mood, especially if they happen to have been tuning in since the very beginning. For others, SNL is a show they haven’t been watching all that long, used to watch, or watched at one time and have started watching again. With its ever-changing cast, writers, sketch concepts...