It's not really a knock to say that nothing in Tropic Thunder is funnier than its first five minutes, so sly that--especially for people watching in theaters--you don't realize right away they are the opening minutes of the movie. This outrageous comedy begins with a series of fake previews, each introducing one of the main characters in the film-proper (not that there's anything proper about this film) and each bearing the familiar logo of a different motion picture studio: Universal, DreamWorks SKG, et al. Such playing fast and loose with corporate talismans verges on sacrilege, but it's an index of how much le tout Tinseltown endorses the movie as a demented valentine to itself. The premise is that the cast of a would-be "Son of Rambo" movie shooting in some Southeast Asian jungle get into a real shooting war with drug-smuggling montagnards. Don't ask--though the movie does have an answer--why such highly paid, usually ultra-pampered personnel as superhero Tugg Speedman (Ben Stiller), Mozart of fart comedy Jeff Portnoy (Jack Black), hip-hop artist Alpa Chino (Brandon T. Jackson), and five-time Oscar-winner Kirk Lazarus from Aus-try-leeah (Robert Downey Jr.) should be running through the jungle unattended and very vulnerable. It matters only that the real-life cast has a high time kidding their own profession and flexing their comedic muscles. Bonus points go to Stiller for co-writing the script (with Justin Theroux) and directing, and to Downey, brilliant as a white actor surgically turned black actor for his role and utterly committed to staying in character no matter what ("I don't drop character till I done the DVD commentary").