For a movie out to lampoon Hollywood excess and vanity, the new action-comedy “Tropic Thunder” has benefited from advance buzz of a time-tested titillating sort. Hype was seeded months ago with reports of Robert Downey Jr.’s postmodern blackface as an Oscar-chasing Method actor (quel edge!) and of Tom Cruise’s unflattering “comic turn” as a no-holds-barred producer (what healthy self-deprecation from a totally normal and self-aware screen idol!).

Fortunately, though, despite its billboard-wide targets, “Tropic Thunder” pulls off some fun satire by committing to the ridiculous foibles of its self-absorbed characters. For the jungle-stranded cast of the film within the film — a high-decibel, on-location Vietnam flick that gets derailed — movie clichés, star personas, and producerly machinations are entertainingly presented as venal lunacy that doesn’t readily loosen its grip on the actors’ reality.

The hapless headliners (introduced by a cacophonous barrage of fake trailers and one ad) are familiar types from the dream factory. Ben Stiller plays the pitiful action star Tugg Speedman, who has gotten lost in his own indistinguishable CGI-larded action sequels. He’s desperate for the respect owned by Mr. Downey’s Russell Crowe-esque Kirk Lazarus, an Australian Oscar fixture who preaches technique to anyone who will listen. Jeff Portnoy (Jack Black) is the drug-fueled, self-loathing star of fart comedies, and Alpa Chino (Brandon T. Jackson) is a dutifully obscene hip-hop star who flogs a drink called Booty Juice.

After the shooting of a customarily massive, high-explosive scene fails spectacularly, the high-maintenance cast is airlifted to the boondocks for a different approach. In pursuit of something true and real, they’re supposed to perform virtually alone under the watchful eye of concealed cameras strung in the trees. But when their incompetent, budget-busting director (Steve Coogan) makes a sudden exit, the jostling egos must fend for themselves. Naturally, the Asian actors playing their Vietcong rivals find a real-life correspondence in a drug gang led by a shrill 12-year-old kingpin (Brandon Soo Hoo). Hovering in the wings are a gung-ho special-effects maestro (Danny McBride) and Nick Nolte’s grizzled Vietnam memoirist, who is a “consultant” on the movie.

As the actors backbite their way through the jungle, back in Hollywood, producer Len Grossman (a balding, hairy-armed Mr. Cruise) schemes evil ways to redeem his studio’s investment, and Tugg’s agent (Matthew McConaughey) works obliviously to secure his client’s all-important contract perks (namely TiVo on the ground). Considering the spotty premise of a production falling apart and pampered actors simply trying to survive, “Tropic Thunder” remains surprisingly entertaining, even when Tugg’s capture ironically sets the plot on the track of the production’s original rescue concept.

But the main attraction is indeed Mr. Downey’s commitment to Lazarus’s commitment to an unworkable proposition, spouting a self-regarding actor’s conscientious hodgepodge of slang and attitude, making the endeavor all the more ridiculous. Lazarus is wisely balanced by being regularly called out by Alpa Chino, who can’t believe the film’s best black role has gone to a white Australian. Reality checks also arrive in the form of Kevin Sandusky (Jay Baruchel), a newbie on the Hollywood scene who isn’t yet drunk on fame. Meanwhile, in a stirring demonstration of non-heroism, Portnoy’s howling fits of agony after he loses his drug stash drive him to steer the group toward the real-life heroin gang in the jungle.

“Tropic Thunder” is the first movie Mr. Stiller has directed since “Zoolander” in 2001, but he’s plowing the same territory of pop-culture bombast and self-abasement that garnered him attention with “The Ben Stiller Show” in the early 1990s and in the Farrelly brothers’ “There’s Something About Mary” in 1998. (The Tom Cruise fixation, which here yields a one-note performance, predictably intense and flat, dates back to even before the TV show.) It’s a style that seems fairly durable given the ever-permuting forms of celebrity worship and sacrifice, which are briskly tweaked here through a glib “E!”-style show.

Spiffed up by co-writers Justin Theroux and Etan Cohen, Mr. Stiller’s tendencies are freshest in “Tropic Thunder” when the characters push past the point of delusion or flirt with being blatantly unpleasant instead of likeably goofy. Most of the time, however, “Tropic Thunder,” though funny enough, falls short of a distantly lurking potential to be genuinely scabrous or nihilistic, as it might have in the hands of two other Fox comedy veterans of the ’90s, Bob Odenkirk (who wrote for the “Ben Stiller Show” as well as HBO’s “Mr. Show With Bob and David”) and Chris Elliott, whose character on “Get a Life” displayed, like the pathetic Tugg, an unquestioning belief in movie and television conventions.

“Tropic Thunder,” amply supplied with pounding hip-hop and parodically clichéd orchestral and classic-rock cues, is as loud as the amped-up, dumbed-down war movies it impersonates. The intestinal gore of “Saving Private Ryan” also gets a nod, as does a checklist of films ranging from “Rambo” to, I believe, “Dangerous Liaisons.” Rather like its wayward fictive platoon, the movie’s structure often barely holds together, and it’s a wonder it doesn’t feel more incoherent. But for all the familiar digs at pompous Hollywood actors, it is the sparkling efforts by stars such as Messrs. Downey and McConaughey that keep “Tropic Thunder” rumbling along.