Ben Stiller is not a subtle guy. On his short-lived, self-titled Fox comedy series in 1992, Stiller oversaw some terrific satire, but the style was often broad. In the decade since he became a household name in There’s Something About Mary, Stiller has drifted primarily toward two comfortably generic personas: the tightly wound, put-upon Everyman (Meet the Parents) and the preening buffoon (Zoolander). All the world loves a clown, and Stiller is content to be one, nuance be damned.

Tropic Thunder, however, finds Stiller as star, co-writer and director attempting a skewering of Hollywood– a genre that requires a scalpel, where he favors blunt instruments. The set-up casts Stiller in buffoon mode as Tugg Speedman, an action-film star whose box-office clout is running out of steam and whose attempt at “serious actor” respectability– playing mentally challenged in Simple Jack– flopped miserably. He’s leading the ensemble in a Vietnam War drama titled Tropic Thunder, along with co-stars– including multi-Oscar winner Kirk Lazarus (Robert Downey Jr.), a Method maniac who had his pigment altered to play an African-American soldier; and heroin-addicted comedy actor Jeff Portnoy (Jack Black)– who are causing just as much trouble for rookie director Damien Cockburn (Steve Coogan). So Cockburn and the movie’s grizzled technical advisor (Nick Nolte) decide to drop the actors into the jungle for a more guerrilla-style filming technique– where they promptly encounter real danger in the form of a well-armed drug operation.

The opening 10 minutes hold out the promise of one of the year’s most hilarious comedies, including a trio of faux movie trailers introducing the preferred milieu for each of the three principal actors. Lazarus and Tobey Maguire make cow eyes at each other as closeted gay monks in Satan’s Abbey; Portnoy plays multiple flatulent, fat-suited characters in a vicious swipe at Eddie Murphy. Even the early moments of over-the-top violence from the movie-within-the-movie– a bayonet-slashed private (Jay Baruchel) tries to gather his intestines– are funny in context. Give Stiller a full-length feature of cinematic parody sketches, and it’d kill.

But as soon as Tropic Thunder enters the “real world” behind the scenes of “Tropic Thunder,” things start to spiral out of control. Speedman wanders off on his own and turns into a combination of Col. Kurtz and Sunset Boulevard’s Norma Desmond when he’s forced to perform a live version of Simple Jack as a prisoner of drug dealers. Tom Cruise puts on a bald cap and hairy forearms to play the hard-ass studio boss like an over-enthusiastic Saturday Night Live host. What begins as a jab at pampered celebrities soon becomes a collection of “bits,” giving everyone their look-at-me moments.

Meanwhile, performing in another movie entirely is Downey as the Australian actor who refuses to break character as a black 1960s grunt. The risky blackface stunt doesn’t go uncommented-on, but Lazarus may be the one character here who’s actually a character, rather than a caricature.

Downey alone is good enough– along with the opening salvo– to save Tropic Thunder from real catastrophe. But by the end, the whole thing has become kind of a mess: a joke on movie-industry bombast that falls victim itself to the idea that a louder gag must be a better gag.

TROPIC THUNDER (2½) Directed by Ben Stiller. • Starring Ben Stiller, Jack Black and Robert Downey Jr. • R, 107 min. • At Century Cinemas Del Monte Center, Lighthouse Cinemas, Maya Cinemas, Northridge Cinemas.