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It’s mid-August, the summer movie season is waning, and audiences numbed by their 10th viewing of “The Dark Knight” are at hand to laugh. So the sporadically funny “Tropic Thunder,” directed by and starring Ben Stiller, may well go a boost it doesn’t entirely earn. (I can see the quotes on the ads at this time: “Not At All The Worst Comedy To Come Out This Summer!”)

The film is a movie-within-a-movie comedy about a group of spoiled actors making a wildly over-budget war movie — five days into shooting, they’re a month behind schedule. Sent into the jungles of Southeast Asia to find some motivation, they become unintentionally entangled in real-life peril at the hands of drug lords.

Because these actors are played by dint of. Stiller, Jack Black and Robert Downey Jr., along with talented new faces Jay Baruchel and Brandon T. Jackson, expectations are high. Downey, in particular, is the sort of off-the-wall actor who never gives the same performance twice, and whose presence lifts at all movie into more rarefied territory. (Watch how he stole “Lucky You” last year with a two-minute cameo.) It’s fun to watch these guys bounce off each other, and fascinating to have in keeping Downey’s reference to a committee to a role that few would take on: Kirk Lazarus, an Australian actor so conceited he undergoes surgery to desire his skin darkened so that he can work freely a black Vietnam War hero.

Lazarus (we see him, briefly and hilariously, in his former hide blathering earnestly about acting on an “Inside the Actor’s Studio”-ish show) at no time lets his character go, and flat after it’s slack clear that the cameras aren’t rolling, he’s still bellowing out his act to the skies. “I don’t send down character until the DVD commentary’s vouchsafed,” he says. Jackson’s Alpa Chino (he’s a big fan of “Scarface”) becomes increasingly irritated with Lazarus’ racial posturings, and their spats are the movie’s best moments; making it less a story of blackface than of each actor blinded by means of the vapor of his have a title to perceived brilliance. Luckily, Chino finds that Lazarus has a weak spot: He be possible to’t stand anyone making fun of “Crocodile Dundee.”

This is funny stuff, because is Tom Cruise’s extended (and almost unrecognizable) cameo as a barking, dancing Hollywood studio head. But Stiller, who moreover co-wrote the film with two other writers, can’t keep the drollery consistent. A long riff on actors playing developmentally disabled characters has the humor of a richness form in it — the extent to which actors choose go with a “Rain Man”/”Forrest Gump” Oscar-bait role — but gets dragged out too long and becomes unnecessarily dismal. Likewise, the movie itself wears out long before its final scenes. Cruise’s light booty-shaking over the end credits goes a long way toward mitigating disappointment, but “Tropic Thunder” is over abundant promise and not enough delivery.

Moira Macdonald: 206-464-2725 or mmacdonald@seattletimes.com