Tropic Thunder - Ben Stiller as Tugg Speedman and Robert Downey Jr as Krik Lazarus

This no-holds-barred send-up of Hollywood pomposity and folly, directed and co-written by Ben Stiller, flaunts its tastelessness and flouts taboos with a brazen recklessness that has already provoked appalled denunciations from the PC police.

Stiller himself stars as one of a bunch of egotistical Hollywood actors who are attempting to make a Vietnam war epic, based on a memoir by grizzled ‘Nam veteran Nick Nolte, on location in southeast Asia. With the movie’s budget spiralling out of control (think Apocalypse Now), the project’s way-out-of-his-depth British director Damien Cockburn (played by Steve Coogan) decides to take his pampered stars deep into the jungle – without any coddling backup – to complete the film “guerrilla style” (think Blair Witch Project).

Tropic Thunder - Kevin Sandusky (Jay Baruchel), Alpa Chino (Brandon T Jackson), Tugg Speedman (Ben Stiller), Kirk Lazarus (Robert Downey Jr) and Jeff Portnoy (Jack Black)

Cockburn’s thesps are ill equipped for the task. Stiller’s Tugg Speedman is the pumped-up hero of a waning series of post-apocalyptic “Scorcher” movies. His bid to catch the attention of Academy voters, however, by playing a buck-toothed, freckle-faced, mentally impaired farm boy in a film called Simple Jack has backfired spectacularly.

Robert Downey Jr’s Kirk Lazarus is a multi-Oscar-winning Australian method actor (think Russell Crowe) who is so hard-core in his approach to his craft that he has gone through a surgical “pigmentation-alteration procedure” to play the movie’s role of a black army sergeant. “I don’t break character till the DVD commentary,” he says, much to the dismay of his African-American co-star, hip-hop singer turned drinks mogul Alpa Cino (say it).

Then there is Jack Black’s Jeff Portnoy, the coke-addled star of a gross-out comedy franchise called “The Fatties” (think Eddie Murphy), who is trying to branch out from his series’ fat-suits and fart jokes. Rounding off the team is Jay Baruchel’s young tyro Kevin Sandusky, the least self-absorbed of the lot.

Inevitably, Cockburn’s plan soon goes pear-shaped and the actors stumble upon a band of local drug lords, who mistake them for DEA agents. The stars, however, initially think the opium gang, bristling with all-too-real weapons, are all part of the movie…

As it directs its satirical fire, Tropic Thunder aims a blunderbuss at its targets, an approach which delivers big laughs but which has already got the movie into trouble. Stiller’s portrayal of Simple Jack, and the movie’s use of the impermissible word “retard”, has attracted particular flak. It’s clear to all but the humourless, though, that the movie isn’t making fun of the disabled. The butt of the joke is the type of Hollywood actor who seeks Oscar glory by playing the disabled. (“Never go full retard,” counsels Lazarus to Speedman, contrasting Sean Penn’s failure in I Am Sam against the success of Dustin Hoffman in Rain Man, Tom Hanks in Forrest Gump and Peter Sellers in Being There.)

Similarly, Downey Jr’s performance as the blacked-up Lazarus isn’t a latter-day minstrel show but an over-the-top parody of white attitudes to – and appropriation of – black culture, as well as thespian self-importance.

Tropic Thunder doesn’t keep up the same level of comic invention all the way through, and seems at times to want to become the very kind of action movie it is ridiculing. But there are moments when the film really is hysterically funny, not least when a certain Hollywood star, heavily disguised by prosthetics, appears on screen as a foul-mouthed belligerent megalomaniac producer.