It was back in March that Tropic Thunder first made it onto my radar. I was skimming through a copy of Entertainment Weekly, and found a full-page picture of Ben Stiller, Jack Black and Robert Downey Jnr in combat fatigues, rifles at the ready, creeping through the brush in a jungle setting clearly meant to represent Vietnam. What struck me, first, was the idea of these excellent comic actors making a Vietnam spoof could be a brilliant wheeze; secondly, the rather jaw-dropping fact that Downey was in blackface.

Hopefully, you’re already aware of Tropic Thunder – it not, you can see the trailer here. Directed and co-written by Stiller, it’s a film about a bunch of hopelessly narcissistic actors shooting a Vietnam movie that goes disastrously wrong and the actors find themselves caught up in a real war. The characters are all extremely funny send-ups of Hollywood archetypes: Stiller plays Tugg Speedman, a Stallone-esque action hero on the wane; Black is Jake Portnoy, a fart gag merchant modelled on Chris Farley; Downey is Kirk Lazarus, a multi-Oscar winning Method actor.

Amping-up Tropic Thunder’s film-within-a-film conceit, you can even find fake websites for Speedman, Portnoy and Lazarus (the Lazarus one is a hoot…). Meanwhile, iTunes are hosting a Hearts Of Darkness-style making of doc, Rain Of Madness, which takes the meta-textual fun to another level.

The movie’s picked up a lot of flack in America, surprisingly, though, not from the Afro-Caribbean community, who seem – at time of writing, at least – not to have vocalised any qualms with Downey’s blackface make-up. The complaints have come from a coalition of 22 disabilities groups, who’re calling for a boycott of the film. Their criticism lies with Tugg Speedman’s previous movie, an Oscar-baiting drama about a man who overcomes his learning disabilities, called Simple Jack. Simple Jack riffs on the cynicism of actors who chase award glory by playing disabled characters; it also serves to demonstrate how little Speedman knows about life outside of the Hollywood bubble. Speedman and Lazarus repeatedly use offensive language to describe the movie and Jack himself, but the point is to amplify their cluelessness and crass insensitivity. As with everything else in Tropic Thunder, the joke is at the expense of the characters. Certainly, the idea that Lazarus is so keen to immerse himself in the role of the platoon’s Afro-American sergeant he’ll surgically change the colour of his skin is directed at how pompous certain actors can be.

Anyway, Tropic Thunder opens this weekend in America; and the question is whether it can finally knock The Dark Knight off the top spot at the box office. Interestingly enough, perhaps, with all this talk of Heath Ledger receiving a posthumous Oscar nomination for his portrayal of The Joker, my money would be on Downey — who’s astonishing career renaissance of late deserves to be rewarded.

Over here, we’ll have to wait a little longer for Tropic Thunder — it opens in the UK on September 16. I’ve reviewed it for the next issue, and I’ll post the full review here nearer the time.