The Boy Friend
In 2005, I ended up directing this musical in a high school as part of a maternity leave cover. I was never quite sure if the rest of the people involved realized that it was somewhere between a pastiche and a parody of 1920s musicals. Either way, it ended up being a good time (and I even had to sub for a student who slipped into a fugue state at the last minute). Yep.
When I first heard that there was a film version, I tried to imagine what it might be like. Would the film crew know they were working with parodic material, or would they try to play it straight?
Now I know. It turns out that they were fully aware of the show's self-mockery, and they leveraged that by making it a parody of 1930s Hollywood musicals. Several famous plots/characters/looks/dance sequences are directly quoted/stolen in the film, which adds a 42nd Street-style framing story that is just a couple of steps removed from the antic silliness of the onstage show.
Although this was unlikely supermodel Twiggy's big-screen vehicle, it is (unsurprisingly) Tommy Tune and Glenda Jackson who shine in minor parts. That said, Twiggy is surprisingly competent (although in an intentionally bad performance, it's hard to know for sure).
And so, this movie goes into the Waiting for Guffman category of meta-theatrical movie comedies. And, really, that's the only way they could have pulled it off.