In The Office (US version) it is revealed that the mockumentary the viewer watches does in fact exist inside the universe. Dunder-Mifflin crew is even interviewed as stars of the show and Pam has a thing for the boom mic operator who worked on the set.
Netflix’ Tick tick boom is based on an eponymous theatre play, which told the background story of the shows creative process. Its main character is a fictional version of the creator, Jonathan Larson.
Season 4 of the Arrested Development revolves around Michael getting release rights from his family members to create a show based on their story from seasons 1-3. He even hires a fictitious version of Ron Howard, who is the real show’s director.
Marvel’s She-Hulk ends with the main character discussing the corners cut during production with a fictionalised version of Kevin Feige.
These are for examples that I could come up with from the top of my head. Can you think of any other?
Didn’t Stargate have a show inside the show called Wormhole Extreme or something like that? And Peter DeLuise who directed Stargate was the director on the fake show as well.
Spike Jonzes’s Adaptation shows Charlie Kaufman writing the screenplay for the movie in itself.
I thought it was interesting that Modern Family was sort of presented as a reality show, but as far as I can remember, they never showed any of the crew/interviewers.
The Japanese film One Cut of the Dead: I don’t want to explain too much, but if you want a movie that takes this concept to the furthest degree then you should check it out.
One Cut of the Dead is one that comes to mind, you can watch the zombie movie at the start, and then the rest of the movie is the crazy things that went on in the background to make the movie. It’s a lot of fun.
Oh, also the Human Centipede trilogy. I haven’t seen it, but my understanding is
spoiler
the second film has the first one in its universe, then the third one includes the second film and immediately leads up to the first one again
The third one also includes the director of the series, Tom Hopper, being invited to see the “real life” human centipede, only to be horrified by what his movies inspired in “real life.”
Not exactly what you asked for, but Dr. Horrible’s Sing-Along Blog had a director’s commentary track that was also a musical.