As “Saturday Night Live” heads toward its 50th anniversary, there is a behind-the-scenes movie in the works about the making of the comedy show’s first broadcast, on Oct. 11, 1975. Called “SNL 1975,” it will be co-written and directed by Jason Reitman of “Up in the Air,” “Juno,” and “Ghostbusters: Afterlife,” and Hulu’s “Casual.” Reitman and cowriter Gil Kenan have based their screenplay on interviews they’ve conducted with surviving original cast members, writers, and crew. Your TV GPS, Globe TV critic Matthew Gilbert’s look at the week ahead in television, appears every Monday morning on BostonGlobe.com. Today’s column covers Jan....