Twenty years ago, "Tropic Thunder" would have been too much of an insider comedy, accessible only to the most knowledgeable of Hollywood devotees. But as writer/director/star Ben Stiller has pointed out in interviews, nowadays everyone's an insider. The inner workings of show business are displayed on the Internet for the whole world to see. Savvy movie fans know about Oscar campaigns and tyrannical studio bosses and self-absorbed actors, and a movie like "Tropic Thunder" -- which hacks Hollywood to pieces more astutely, mercilessly, and hilariously than any satire in at least a decade -- can emerge as one of the year's best comedies without going over everyone's heads.

It begins with fake trailers introducing us to the fake actors in the movie. Tugg Speedman (Stiller) is an action hero who recently made an ill-advised grab for Oscar glory by playing a mentally handicapped man in "Simple Jack" (think Sean Penn in "I Am Sam"). Jeff Portnoy (Jack Black) is known for his flatulent family comedies in which he plays all the roles (think Eddie Murphy in everything), though his personal life is a heroin-flavored mess. And Kirk Lazarus (Robert Downey Jr.) is a five-time-Oscar-winning Method actor known for fully immersing himself in his characters. "I don't read the script, the script reads me" is one of the pretentious, nonsensical things he likes to say.

These three Hollywood airheads are the stars of a big-budget Vietnam epic being directed by Damien Cockburn (Steve Coogan), who quickly finds that their egos and pampered lifestyles are interfering with the production. (We're told that the film is "one month behind schedule after only five days of shooting.") The studio head, Les Grossman (Tom Cruise), a vulgar despot with hairy arms and a gold chain around his neck, is furious, demanding that Damien get the film back on track or heads will roll.

As it happens, Four Leaf Tayback (Nick Nolte), the battle-scarred Vietnam veteran whose memoirs the film is being based on, is on the set as a consultant, and he has an idea for Damien: Drop the actors into a real Vietnamese jungle, with no cell phones or other comforts, and shoot the movie with cameras hidden in the trees. The Hollywood phonies' reactions to all the deprivations of war will seem a lot more real, and the actors will be forced to focus on their work.

Did you guess that once they've been put in the jungle, the stars encounter actual bad guys who they think are fellow actors? And that they're surprised when the villains seem to be firing actual bullets? Yes, that's the initial concept, but thankfully it isn't long before the reality of the situation becomes obvious to them and the film shifts to its new direction: the actors have to rescue Tugg from an Asian heroin factory run by a ruthless teenage warlord (Brandon Soo Hoo).

"Tropic Thunder" (that's the name of the Vietnam drama they're making, too) is packed with ingenious running jokes skewering the behind-the-scenes stories that movie fans know so well. Tugg Speedman is a well-meaning but vain idiot (an area of expertise for Stiller), while Jeff Portnoy is suffering from heroin withdrawal during much of their jungle trek. Kirk Lazarus, the Method actor, is playing an African American soldier, so he has had his skin chemically darkened -- which means, yes, Robert Downey Jr. is in blackface for most of the film. And not just blackface, but blackvoice, too, because Kirk Lazarus never breaks character. Heightening the tension is the fact that there's an actual black guy in the movie with them, a rapper-turned-entrepreneur named Alpa Chino (Brandon T. Jackson) who's trying to break into acting. You may rest assured that he and Kirk Lazarus have some conversations about race.

The fifth member of their team is an eager young actor named Kevin Sandusky (Jay Baruchel). Without a gimmick, this character appears to be superfluous, until you realize it's his normalcy that makes him important -- he's the straightman surrounded by crazies. Stiller and his co-writers, Etan Cohen (TV's "King of the Hill") and actor Justin Theroux, know comedy well enough to appreciate the importance of such a character in a loopy scenario like this one.

All of the supporting characters are fantastic, too (up-and-comer Danny R. McBride scores again as the production's pyrotechnics expert), and the casting is perfect. Nick Nolte as a grizzled veteran who sleeps in a tent ("Beds give me nightmares")? Matthew McConaughey as Tugg's tooly agent Rick Peck, who calls everybody by nicknames, including himself ("the Pecker")? And Tom Cruise as the terrifyingly foul-mouthed studio head? Genius, sheer genius -- especially for Cruise, who might wipe away all the negative impressions people have had of him over the last three years with this single performance.

This is the first film Stiller has directed since 2001's "Zoolander." He's acted in some bad comedies since then, but "Tropic Thunder" is proof that he hasn't lost his edge, neither as a filmmaker (he's impressively disciplined here) nor as a performer. It's no surprise that Stiller, the son of Jerry Stiller and Anne Meara and a Tinseltown insider since birth, should have such keen insight into the vanities of Hollywood. His short-lived sketch series, "The Ben Stiller Show," had glimpses of it. "Tropic Thunder" feels like it represents everything he and Downey and Black have learned in their years of making movies -- and it shows that they're self-aware enough to realize how silly the whole profession is.

Grade: A-

Rated R, pervasive harsh profanity, a lot of graphic sexual dialogue, abundant graphic violence (played for laughs)

1 hr., 47 min.

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