Ben Stiller, above left, Robert Downey Jr., above right, and Jack Black, below, play actors caught up in an unexpected real-life conflict in the comedy Tropic Thunder.

The freewheeling comedy Tropic Thunder, about a crew making a war movie in Vietnam and unwittingly getting caught up in a real life-or-death adventure, starts off like a house afire.

Ben Stiller, who directed and co-wrote the film as well as starring in it and producing, introduces the principal characters in a wacky series of commercials and movie trailers spoofs. Stiller himself is Tugg Speedman, an action movie star who has made six world-on-fire films as the Scorcher and is now desperate to find another niche because the Scorcher character is fizzling and Tugg’s embarrassing attempt at serious drama tanked.

Also looking for a fresh start in what everyone hopes will be the epic war movie to end all epic war movies is Jack Black as Jeff Portnoy, a drug-addicted star whose success has hinged on a series of gross-out movies called The Fatties. Kirk Lazarus (Robert Downey Jr.), a serious Oscar-winning actor, seeks new challenges in Tropic Thunder, for which he has undergone a pigment transplant to make him look like an African-American. Real African-American hip-hop star Alpa Chino (Brandon T. Jackson) just wants to reach beyond his silly stage persona with this film, while newcomer Kevin Sandusky (Jay Baruchel), so new that none of his fellow cast members ever gets his name right, just wants to be part of this classic film.

Tropic Thunder’s tempo roars on as first-time director Damien Cockburn (Steve Coogan) finds himself under the gun from big-time producer Les Grossman (a hilarious and nearly unrecognizable off-the-wall Tom Cruise) who rages at him over the phone from Hollywood for going way behind schedule and way over budget. Egged on by real-life Vietnam war hero John “Four Leaf” Tayback (Nick Nolte), Damien agrees to a last-ditch effort to save the film and his career by making their war movie all too real for his stars. He hopes to get the performances he has been looking for by sending them into the jungle alone to act out their scenes in front of remote-controlled cameras. He believes they will be much better if not distracted by the assistants and air-conditioned trailers and food spreads and the TiVo that Tugg’s agent (Matthew McConaughey) is trying to secure for him.

It sounds like an innovative and sound idea. But things quickly turn disastrous as the guys are set upon by a team of drug gangsters who call themselves Flaming Dragon. They think the movie stars are narcs and try to kidnap and/or kill them.

That’s the very funny, prolonged setup for Tropic Thunder which, unfortunately, doesn’t live up to expectations for its middle section. It’s the film’s weakest link. The midsection is only fitfully amusing, more often stretching to pull laughs from situations that seem not all that funny. There’s a lot of sitting around campfires, trekking through the jungle and downright silliness in the Flaming Dragon compound, whose top dog turns out to be a child. When one of the guys is kidnapped by Flaming Dragon, the others of course attempt to rescue him, leading to predictable mayhem.

Happily, however, once all that is dispensed with, Tropic Thunder gets back in stride with a very funny, very satisfying final 20 minutes which, again, spoofs the big Hollywood players. Cruise is especially funny as Les, a bald-pated, self-aggrandizing mogul with a furry chest and forearms, who shouts commands to his underlings, expecting immediate results. Cruise has never been looser or more outrageous in a movie. It should earn him an Academy Award nomination.

The film itself won’t win one, but there are plenty of funny touches by Black, Coogan, Downey, Nolte and Jackson to keep things humming.

This is the closest in his directing that Stiller has come to his outlandish cult hit Zoolander, in which he played a jealous male model. Stiller is buffed to the nines for the role of Tugg Speedman, a man who takes himself very seriously. Good thing that Stiller himself does not.

*** 1/2Tropic Thunder

Starring: Ben Stiller, Jack Black, Robert Downey Jr., Steve Coogan, Tom Cruise, Matthew McConaughey, Jay Baruchel, Danny McBride, Brandon T. Jackson, Bill Hader, Nick Nolte.

Rated: R, contains violence, profanity, drugs.

mjanuson@projo.com