Ben Stiller hasn’t directed a film since Zoolander in 2001, but this rumbustious farce, taking aim at Hollywood and its denizens, should easily trump that doubtful ace if only because of its starry cast.

This includes an uncredited Tom Cruise as Len Grossman, the kind of parody studio boss we love to see: foul-mouthed, big-gutted and instructing one of the junior crew members to punch a recalcitrant actor in the face. You barely recognise him.

But there is also Stiller himself, Robert Downey Jr, Jack Black, Nick Nolte, Steve Coogan and Matthew McConaughey. Not surprisingly, the budget came in at $100 million. 

This lot are dropped in the jungle somewhere in South-East Asia to make a film about Vietnam that will blast Platoon and Apocalypse Now clean out of the water. They are, however, incapable of acting it out under the circumstances, since they are directed by Coogan’s English film-maker, who is little more than a well-spoken idiot, and assailed by drug runners who are even more dangerous than the Viet Cong they are supposed to be fighting.

Downey has the riskiest part: he plays an Australian star who has undergone skin pigmentation to play a black American. Jack Black is the most obvious parody as a kind of retarded fatty bouncing around in his soldier’s uniform. The day he doesn’t over-act, I’ll eat my copy of the publicity material.

Tropic Thunder is supposed to be about narcissistic actors, imbecilic visionary directors and the whole Hollywood mixture of ballyhoo and idiocy. But it covers its options by including long scenes of shoot-’em-up action, including deliberately clichéd slow-motion shots of death and destruction and buddy-buddy sequences when GIs comfort their dying compatriots with words calculated to bring tears to the eyes of morons.

Strangely, though, the funniest part of this rip-roaring, determinedly over-the-top movie comes right at the beginning, when a series of phoney trailers almost persuade one that they are the real thing.

There’s Scorcher VI: Global Meltdown, starring Stiller’s Tugg Speedman; The Fatties: Fart 2 with Black, and Satan’s Alley, a terrible art movie starring Downey and Tobey Maguire. They’re short and certainly sweeter than the noisy, action movie farce that follows.

As for the rest, good jokes follow bad in quick succession, and there’s no doubt that some will find them more hilarious than I did. But it’s really not all that risky, since Hollywood is such a soft target to be satirised this way. The real place is much more subtle, showing considerable politesse and dispensing buckets of flattery while carefully undermining unusual or risky projects.

Tropic Thunder gives us exactly what we expect of it — lashings of easy fun, spare-no-expense action, actors delightedly pushing the boat out in all-out parody and a dig at the absurdity of Hollywood that the big boys back in LA will find quite comfortingly obvious.

But subtle it isn’t, even though it clearly thinks it is.