Three new professions are added. Actor, Director and Writer.
These dwarves make movies, producing projector film that can be used alone to watch the movie inside. Just don't ask how, like how you don't ask how dwarves roast stuff without fire and brew without water.
These movies are, of course, generated entirely procedurally, and almost all of them completely lack any real artistic value, and are only useful as cheap trade goods.
But I can already play America Simulator without even turning on a computer.
Okay, same idea. Except while they create movies, including film reels, there are no actual film projectors in the game so they can't actually be watched. They're just considered crazy people. However, they will still demand guilds be built where they will Simulate Projector endlessly and pretend to be actually watching them, then they'll argue about what they imagine they just saw, leading to another more useless professions, Film Critic, Projectionist, Producer, Ticket Taker, Talent Agent, Popcorn Vendor, Gaffer, Best Boy, and others.
Allow these to continue existing and they will replace all other professions, leading to the death of the fort by starvation as no actually useful jobs get done.