I’d been a bit wary of Tropic Thunder based on the reviews I’d seen, but it turns out I needn’t have worried. I suspect that the reviews in question were written by slightly snooty types more concerned about what they think they should laugh at rather than people who were laughing as fully, as honestly, as the audience in the screening I saw.

That critical response reminds me of Alexei Sayle’s observation that in playing for left wing audiences (he played a lot of political benefits) there was often a lag before laughter as people assessed whether they were allowed to laugh at what they’d heard and seen. So, let’s be blunt: there is humour here that could cause offence to those with particularly precious attitudes to disability, and race. But the film’s makers — it’s helmed by co-writer/director/star Ben Stiller — are fully aware of those responses and incorporate them into the movie.

Anyone choosing to be offended is acting out of kneejerk response rather than considering the more complex messages that the film actually contains. For instance, disability groups are not happy about the way the film uses the word ‘retard’. A shame, since the script contains a hysterical and perceptive sequence about the portrayal of learning difficulties in Hollywood films. Oh, and that speech is delivered by Robert Downey Jr, who is blacked up for the bulk of the action — again, with genuinely hilarious consequences, as his portrayal of a Method-obsessed actor is mercilessly dissected by a co-star who really is black.

The film’s primary target is Hollywood itself. A group of actors are filming the war epic to end all war epics in Vietnam, and under director Steve Coogan find themselves a month behind schedule after five days of shooting. Coogan decides to abandon the grandiose approach he’d intended and instead opts for an improvised shoot, letting his stars loose in the jungle and filming the action from hidden cameras. Only, things go wrong of course. The actors are spotted by a violent drug gang and mistaken for operatives of the Drug Enforcement Agency, and dealt with accordingly.

It’s all cleverly set up by a bunch of mock-trailers for films that its cast of supposed heavyweights are involved with. Ben Stiller’s promotes his sixth outing as the star of the science fiction action Scorcher films. Jack Black is fatsuited in a sequel about a group of overweight ass-gas-passers, in which he plays every family member. And Downey Jr is a medieval priest, tormented by forbidden love for a fellow man of God. Every trailer is note-perfect, from music to product placement opportunities, and sets the audience up for the variety of what’s to come.

Tropic Thunder is the funniest film I have seen in a long time. Jack Black, Nick Nolte and Tom Cruise are among other big names joining Stiller and Downey Jr, and no opportunity is missed for a gag, whether it’s a joke on the vanity of actors, an observation about the portrayal of race, or splendid slapstick from a cast who are all strong physical performers.