Tropic Thunder (15)

For a comedy about a mega- budget Hollywood folly, it's startling how close to being a mega-budget Hollywood folly Tropic Thunder is. Directed and co-written by Ben Stiller, it features Stiller himself as a Stallone-alike action hero who's reduced to appearing in buddy movies alongside Martin Lawrence. Jack Black plays a gross-out comic whose fart-filled films aren't a million miles away from Eddie Murphy's. And Robert Downey Jnr plays an Oscar-winning, hell-raising Australian thesp who might just be based on Russell Crowe.

After some hilarious mock-trailers establishing who they all are, we cut to the set of a Vietnam War epic that the three actors are making together. The production is going so badly that the director, Steve Coogan, is forced to take a new tack.

His Plan B is to use the Blair Witch method of dumping his actors in a jungle rigged with hidden cameras, and telling them to act out the dialogue in situ. The only trouble is that the jungle he dumps them in is the site of a heroin factory, so the people who keep shooting at them aren't extras playing the Viet Cong; they're genuine trigger-happy guerrillas.

It's not a bad idea, having a troupe of molly-coddled actors fighting real battles which they think are staged, but in terms of comic mileage, Tropic Thunder gets only a few yards out of it. Almost immediately, Downey figures out that they're in actual danger rather than movie-danger, thereby stopping the joke before it's got started. The film then zig-zags between various locations and characters, including Nick Nolte's Vietnam vet, and a bald, bellicose studio boss played by Tom Cruise.

With so many A-listers jockeying for position, they all become supporting players with no star at the centre. Downey noses ahead as a method actor who has his skin pigmented to play a black man – funnier than it sounds – and the sporadic bursts of brilliance give audiences their money's worth. But what with all the explosions and helicopters, it's debatable whether the producers got good value. It's a case of big budget, big stars, and only medium-sized laughs.