Making movies about making movies is never easy. Go into too much detail and you risk losing the audience; skimp on the specifics and the idiosyncrasies and absurdities of the movie-making process are lost. Somewhere in the middle of those two extremes stands “Tropic Thunder,” which uses the broadest targets—clueless Hollywood studios and buffoonish, self-important actors—to build a high-concept comedy that scores some surprising hits.

Ben Stiller stars as Tugg Speedman, a major action movie star looking to revive his flagging career. That revival comes in the form of “Tropic Thunder,” a big-budget epic about a Vietnam War rescue mission that goes terribly awry. Speedman’s co-stars are a motley crew of outsized egos and celebrity excess. There’s Jeff Portnoy (Jack Black), a coked-out comedian coasting on the success of his franchise of fart movies; Alpa Chino (Brandon T. Jackson), a hip-hop star looking for some acting cred; and Kirk Lazarus (Robert Downey Jr.), an Oscar-winning method actor so dedicated to his craft that he dyes his skin to play an African American soldier in the film.

With a cast like that, it’s no wonder the film is a month behind schedule after five days of shooting. In an effort to save the film and whip the stars into shape, Four Leaf Tayback (Nick Nolte), the hard-ass Vietnam vet whose memoirs inspired the movie, convinces director Damien Cockburn (Steve Coogan) to drop the dysfunctional cast into the middle of the jungle, a stunt he says will toughen them up and turn them into men. A vicious drug gang operating in the jungle has different plans for the actors.

When “Thunder” is on target, the results are deadly. The mock trailers that show up at the beginning of the movie to establish the characters are spot-on lampoons of ubiquitous multiplex dreck. The best running gag in “Thunder,” and the one that’s drawn the most ire from advocacy groups, is “Simple Jack,” a film about a mentally handicapped farm boy that Speedman starred in so he could get an Oscar. It’s a send-up of Hollywood vanity, bad writing (when sad, Simple Jack says his “eyes rain”), and those crass, calculated “important” movies that always show up a few months before the Oscars.

A few clips from “Simple Jack” and Kirk Lazarus’ brilliantly offensive critique of Speedman’s performance as Jack, would have been funny enough. But Stiller, serving as director and co-writer with Justin Theroux and Ethan Cohen, brings the joke back when Speedman discovers that “Simple Jack” is the drug gang’s favorite movie. This leads to a hilarious sequence in which Speedman, saddled with some fake teeth, a wig and some makeup, must re-enact all of “Simple Jack” under threat of torture. Stiller and his collaborators fully create a movie universe with its own sort of manic logic, and that keeps the film going when it occasionally slows.

The real lynchpin here is Robert Downey Jr. Kirk Lazarus is ostensibly a spoof on Russell Crowe, but Downey also parodies himself and any other method actor who has thrown himself way too far into a role. If anyone can get away with using blackface in the service of satire, it’s Downey. He’s seriously good here, and Lazarus’ conviction that he is, in fact, a black man, is as convincing as it is absurd. Downey walks a fine line between caricature and legitimate role, and the results are awesome.

The rest of the cast isn’t as lucky, though. Jack Black’s turn as a Chris Farley-esque comedian with a raging drug addiction is relatively weak and feels like more of the same old Jack Black shtick. Worst of all is Tom Cruise’s small role as a proudly profane studio head. Cruise is trying desperately to hang with the cool kids and prove to audiences that he’s not just the wacky Scientologist who keeps impregnating Katie Holmes. From the bald wig and fake beard to the various vulgarities and a really terrible dance sequence, Cruise just looks like he’s trying too hard. That wouldn’t be so bad if Downey, Stiller and the rest of the supporting cast (including Jay Baruchel as an eager actor and Danny McBride as the film’s pyromaniac special effects dude) didn’t make it look easier than it is.