Director, co-screenwriter (with Justin Theroux and Etan Cohen) and star Ben Stiller's raucous, hit-and-miss Hollywood satire "Tropic Thunder" is the latest in a long line of movies to demonstrate that, for people who work in cinema, there's nothing funnier than a day at the office. Think of "The Player" or "Bowfinger." "Adaptation." "The Last Shot." "The Last Action Hero." "State and Main." "Get Shorty."

Stiller's contribution to the genre is more broadly accessible than most. Viewers who don't have a geek-level preoccupation with the film industry may not appreciate every joke, but there are plenty of cracks that don't have to be decoded.

There's blood in the water right off the bat, thanks to a fake concessions ad and series of fake trailers featuring Stiller, Jack Black, Robert Downey Jr. and Brandon T. Jackson in character as, respectively, top-dollar action star Tugg Speedman, rowdy knucklehead Jeff Portnoy, Oscar-winning Aussie thespian Kirk Lazarus and booty-wigglin' rapper Alpa Chino.

Two of the previews (featuring Speedman and Lazarus) are smartly funny. The concessions ad (Alpa Chino endorses Booty Juice energy drink and Bust-A-Nut protein bars) is less imaginative. And the trailer for Portnoy's "The Fatties: Fart 2" falls as flat as its fake title.

The ratio of hilarious comedy to dead air in the rest of the movie is higher, especially at first. Speedman, Lazarus, Portnoy and Chino are the stars of a Vietnam War action movie filming in southeast Asia. The production screws up a $4 million explosion. The studio, personified by testy mogul Les Grossman (Tom Cruise), is furious, and British director Damien Cockburn (Steve Coogan) is fed up with his high-maintenance cast.

The screenwriter, war vet "Four Leaf" Tayback (Nick Nolte), tells Cockburn to quit coddling everyone, and take the actors deep into the jungle, where they can shoot the movie "guerilla style." Cockburn catches the vision and rounds up the troops, leading to a funny speech about the new chain of command that has an even funnier immediate aftermath.

The gamble begins badly, however, and things get even more complicated after the fake platoon runs afoul of actual drug dealers.

Sometimes Stiller's out-of-left-field humor works, like when Speedman battles a "ferocious" wild animal. Sometimes it's just weird and distracting, like when the drug dealers turn out to have a vicious 12-year-old boss.

The film's best performance and its funniest material both belong to Robert Downey Jr. Lazarus has dyed his skin to play Sgt. Lincoln Osiris, who's black, and stays in character -- completely -- even when the cameras aren't rolling.

"I don't drop character 'til I do the DVD commentary," Osiris growls. It's a tricky, challenging part, but Downey Jr. nails it.

Then you have the likes of Cruise's Grossman. The actor agreed to look bald and fat, see, but it's actually Tom Cruise! Oh, and he yells profanities, too. Bet you'd laugh if he were a little old lady.

Profanity and vulgarity, incidentally, don't seem to even register with the MPAA these days. "R" has become the catch-all, say-whatever-you-feel-like rating and "Tropic Thunder" is just the latest pointlessly ear-blistering example. Why do so many comedies have to be funny in spite of themselves?

B

Tropic Thunder

Director: Ben Stiller

Cast: Ben Stiller, Robert Downey Jr., Jack Black, Tom Cruise

Running time: 1 hr., 47 min.

Rating: R for pervasive language including sexual references, violent content and drug material

Location: Opened Wednesday at theaters everywhere