Stephen Sondheim was a hot property in 1981. Riding the momentum of Sweeney Todd, his next musical would be Merrily We Roll Along [Wikipedia], and that would be a gigantic flop, closing after over 50 previews, dozens of rewrites, and only 16 performances. The show would be rewritten again a couple of times before this 2012 London production was filmed [2h15m].
True Broadway diehards might enjoy this document, an audience filming of the original 1981 production [1h55m] from sometime in the 2 weeks it ran. The quality is poor, but it exists!
The rewriting process involved the composer and the writer, as told here by Randy West who was doing an early restaging in Phoenix.
Netflix has a terrific 2016 documentary, Best Worst Thing That Ever Could Have Happened [2m30s trailer] made by original cast member Lonnie Price, which documents the original production with current and then interviews and footage. Accompanying that film’s release, here is a Q&A at New York Film Festival [20m], and a cast panel from Broadway Sings [45m].
The story being told backward across 20 years, it seems that making-movies-across-time Master Richard Linklater has begun production on a movie musical that will be complete in time for a 2040 release, allowing the same actors to age from young idealists to bitter middle age, with production starting up ever couple of years to film.