The best stories are self-contained tales. The worst stories rely on the audience knowing something outside of the movie, which is similar to theater where actors break the fourth wall, meaning that they step outside of their story on the stage and engage the audience somehow. Generally speaking, this is not a technique that works very well.
“Galaxy Quest” and “Tropic Thunder” have very similar plots. The basic idea is that a bunch of actors are thrown into a world where they have to work together in a real-life situation that’s no longer about acting. In “Galaxy Quest,” actors from a Star Trek-like TV show get transported on to an alien spaceship to help them fight an alien villain determined to wipe them out. In “Tropic Thunder,” actors filming a Vietnam war movie find themselves fighting a real war.
Although both movies got nearly identical positive scores on Rotten Tomatoes, a movie reviewing site, I personally found “Galaxy Quest” to be funnier and more coherent than “Tropic Thunder.” One big difference is that “Galaxy Quest” has a villain that drives the story from the start while “Tropic Thunder” doesn’t reveal its villain (a heroin warlord) until much later in the film.
Where “Galaxy Quest” spoofs Star Trek, “Tropic Thunder” spoofs various war movies including “Apocalypse Now” and “Platoon” while also poking fun at the silliness of Hollywood movies and how Hollywood works. Because “Tropic Thunder” attacks so many different targets, its humor is less focused than “Galaxy Quest,” which pokes fun at Star Trek. As a result, “Tropic Thunder” seems less funny and more like one big Hollywood inside joke
The characters in “Tropic Thunder” also pursue different goals that don’t seem as integral to their lives. Ben Stiller wants to win an Oscar and he finally does, but achieving this goal doesn’t feel like a grand achievement. Instead, it feels like another one-liner gag and brings a smile to your face, but no emotional sense of completion. “Tropic Thunder” tries valiantly to be funny, but it’s a hit-and-miss story that doesn’t quite match the more consistent flow of “Galaxy Quest.”
If you watch both movies, you can see how the same idea can be implemented in two different ways. The story of actors having to play their roles for real is funny, but after seeing both movies, you can see which one pulls off that idea more successfully.