Having grown up in a showbiz family, Ben Stiller has long had an insider’s perspective on the foibles and follies of the entertainment industry, which has fed projects like his Fox TV sketch comedy show and his last directing effort, the zany 2001 male-supermodel sendup, Zoolander. With Tropic Thunder, Stiller takes direct aim at the egomania, self-absorption and crassness of Hollywood star culture, with sometimes outrageous results, but his satire is ultimately as deep as one of his old three-minute TV skits.

Stiller heads a formidable cast as Tugg Speedman, a top action-movie star whose Scorcher series of thrillers has reached the limits of absurdity and whose attempt to “stretch” with the part of a mentally challenged farmhand has become an industry joke. Jack Black plays Jeff Portnoy, who’s made his fortune starring in a flatulence-filled comedy franchise called The Fatties. (Think Eddie Murphy’s Klump family.) And Robert Downey, Jr. is Kirk Lazarus, an Australian acting powerhouse who’s already won five Oscars in recognition of his intense commitment to his roles. Tugg, Jeff and Kirk are co-starring in Tropic Thunder, a Vietnam War drama based on the memoir of maimed hero “Four Leaf” Tayback (Nick Nolte, ideally cast), and a production spiraling out of control in Southeast Asia under the direction of frazzled Brit Damien Cockburn (Steve Coogan). When foul-mouthed studio head Lee Grossman (Tom Cruise, disguised as a bald, pot-bellied boor) threatens to pull the plug on the movie, Cockburn decides to take his cast deep into the jungle and resume shooting “guerrilla-style.” The pampered actors find themselves in real jeopardy when they cross paths with local drug dealers who don’t like having their turf invaded. All the while, the dimwitted Tugg is convinced his capture and maltreatment are part of an unorthodox filmmaking process.

The script by Stiller, fellow actor Justin Theroux and Etan Cohen includes some knowing insights into the rarefied world of A-list stars and their handlers and how insular their lives and attitudes can become—and how pompous their public utterances can be. The cleverest moment occurs when Cruise’s Grossman takes a phone call from Tran, the 12-year-old drug lord who has kidnapped Tugg, and lashes out ferociously as if the little criminal is poaching one of his stars. (Tugg’s agent, appealingly played by Matthew McConaughey, somehow thinks the crisis is all about the absence of TiVO in Tugg’s suite.)

But too often, Stiller and his co-writers settle for the broad and obvious. Stiller’s Tugg is of a piece with his hopelessly dense Zoolander, so there aren’t too many surprises to be found in his performance. Black’s character comes to be defined by his major drug habit, which quickly becomes a comic dead end. Apart from Cruise’s stunt casting, the most memorable performer by far is the always surprising Downey. Here, the inspired conceit is that the blond, blue-eyed Lazarus has had pigmentation surgery in order to play a black soldier, and is the kind of actor who constantly stays in character, much to the annoyance of his African-American fellow actor, rapper Alpa Chino (Brandon T. Jackson), named in tribute to the sacred Scarface. Downey runs through the minefield of blackface and comes out safely on the other side, with a hilariously gravel-voiced impersonation of one tough brother and one seriously crazed thespian.

For a pretty ludicrous comedy, Stiller has recruited some impressive talent on the other side of the camera, including production designer Jeff Mann (Transformers, Mr. & Mrs. Smith) and Oscar-winning cinematographer John Toll (Braveheart, Legends of the Fall, The Thin Red Line). Tropic Thunder is Stiller’s most ambitious directing project, and it delivers enough high-powered action to fit in well with the summer blockbusters. Audiences who don’t mind “Mad TV”-style humor stretched out to nearly two hours should be sufficiently entertained. And they’re advised not to arrive late, or they’ll miss a high point—trailers for the Tropic Thunder stars’ latest projects, including a Brokeback Mountain-like romance set in a Middle Ages monastery.