It’s always been difficult to satirize the movie business, simply because it’s already so absurd in its natural state. But it’s gotten harder over the last decade or so as the amount of “inside” information on it has grown exponentially year by year, flooding the airwaves, magazines and Internet. It’s hard to believe that there was a time when ordinary Joes didn’t care about the weekend boxoffice race or battles over residuals, but instead talked about the actual quality of the pictures. But even in a more innocent age, Ben Stiller’s “Tropic Thunder” would probably have seemed like pretty puerile stuff. Its targets are so obvious and its take on them so broad and sophomoric that it comes across as bland. Still, its mixture of insider winks and goofiness will probably appeal to a moviegoing public that prides itself in being “in the know” about the biz and, as Will Ferrell’s popularity demonstrates, willing to opt for just about the lowest comic denominator one can imagine. It covers both bases.

The movie begins by introducing us to four ersatz stars via fake commercials and trailers. There’s Tugg Speedman (Stiller), an action heavyweight whose career is stumbling, especially after he tried a disastrous change-of-pace role as a mentally-challenged man. Jeff Portnoy (Jack Black) is a drug-addicted comic tired of making his money on a string of fart-based farces in which he plays multiple roles in fat suits. Kirk Lazarus (Robert Downey, Jr.) is an Australian multiple Oscar-winner known for immersing himself in “challenging” roles, like his new one as a medieval monk with the hots for a young novice. And Alpa Chino (Brandon T. Jackson) is a rapper anxious to get cred as an actor. They’re all on location trying to wrap up filming the eponymous Vietnam war epic based on a book by grizzled vet Four Leaf Tayback (Nick Nolte) and directed by harried Brit Damien Cockburn (Steve Coogan). Watching from afar are foul-mouthed studio chief Lee Grossman (Tom Cruise in heavy makeup) and Speedman’s agent Rick Peck (Matthew McConaughey), who seems most concerned that his boy hasn’t gotten the TiVo his contract promised. And closer are the picture’s fifth star and voice of reason, newcomer Kevin Sandusky (Jay Baruchel), as well as a crazed explosives expert (Danny McBride).

Things go wrong immediately, in both the fictional and real “Tropic Thunder.” In the middle of shooting the big finale, Speedman halts the scene to blather about motivation and his interplay with Lazarus. And then the culminating explosion is set off before the cameras are rolling, a disastrously expensive mistake that brings Grossman on line to berate one and all. The problem with all this is that it’s basically ripped-off from a far funnier movie, Blake Edwards’ “The Party” (1968), in which Peter Sellers played a bumbling Indian actor who ruined a big battle sequence in a “Gunga Din”-like epic by overacting a bit part, and then went on to—you guessed it—set off an explosion before the crew was ready to film it. Stiller has simply transposed the material to a different setting and played it on a more lavish scale. And as a rerun, it’s not nearly as amusing as Edwards and Sellers made it.

“Thunder” then goes off on a different but hardly better tangent as Cockburn’s persuaded to save the movie by sending his stars off alone into the jungle with cameras hidden everywhere to catch them acting out the movie “for real,” as it were. Of course, there’s no realism whatever in this premise, since a viable picture could never be made in such a way. But setting that aside, the remainder of the movie consists of the guys getting lost in the wild, Tugg getting captured by an army of drug merchants led by a young commander Tran (Brandon Soo Hoo) whom he mistakenly takes for part of the plot, his life spared because Tran is a devotee of his “retard” role as “Simple Jack,” and his co-stars eventually stumbling on the camp and hatching a plot to rescue him, action-movie style. A Oscar ceremony coda can’t hold a candle to the one that closed “In & Out.”

There are moments scattered throughout “Tropic Thunder” that are pretty good, but for the most part it’s awfully flat stuff, and the big action finale never takes off. By far the best running gag is Downey’s performance as a guy who’s such a perfectionist that he actually has his skin dyed in order to play a black soldier and never steps out of character, infuriating Chino. Downey takes the joke and runs with it brilliantly; even though he occasionally swallows his dialogue, you hear enough of it to laugh, and he’s been given one of the script’s funniest moments, when he explains to Speedman how he made a mistake in going “full retard” in “Simple Jack”—a decision that Oscar never rewards. (Of course, disabled persons’ interest groups understandably won’t appreciate the use of the word for comedic effect.)

But otherwise the stars do not shine. Stiller’s turn is mostly pseudo-virile poses, except for the really embarrassing scenes in which he has to do his Simple Jack routine, and Black’s shtick is very tiresome. Newcomers Jackson and Baruchel are shortchanged in forgettable parts, but though the harried Coogan comes across as nothing but a younger version of Eric Idle and the abrasive McBride squanders the good will he won in “Pineapple Express,” Nolte and McConaughey brighten things up considerably when they’re on, the former with his patented gruffness and the latter with some unexpected comedic spark. As for Cruise, he’s certainly ferocious, but his studio mogul is really a throwback to the 1940s, and the turn is basically a one-note affair that, even though a cameo, goes on too long, especially in his smug closing dance.

This was obvious a big-budget production, and it looks and sounds it, especially in the pyrotechnic department. But there’s no more imagination to Stiller’s direction or John Toll’s cinematography than there is in the script or (apart from the irrepressible Downey) the performances. “Tropic Thunder” makes a lot of noise, but fails to produce the promised gales of laughter.