Ben Stiller's biggest problem as a director has been that his material has never quite been worthy of his obvious ambition. But in Tropic Thunder, a satire about the insecurity and immaturity of movie stars, which he co-wrote with Justin Theroux and Etan Cohen, Stiller's obvious comfort and confidence in the material grounds the film so firmly that, for the first time, his directorial ambitions can flourish. The premise is that five actors -- three of them international superstars -- are stranded somewhere in Asia believing they are shooting a guerrilla-style Vietnam War epic, when they're in fact caught up in very real danger. This structure serves up so many delicious possibilities that Stiller and his cohorts can't help themselves, they try everything: physical comedy; self-serious Oscar-bait trailers; profanity-laced diatribes from Hollywood power players -- they even mock the horrors of drug withdrawal, all the while playing up the ceaseless insincerity of almost everyone involved in moviemaking. Well-shot by cinematographer John Toll, and cannily edited by Greg Hayden, the film is a visual treat. Moving briskly from joke to joke, Tropic Thunder, much like Hot Fuzz, works as both as an action film and as a spoof of action films. When there are visual allusions to other Vietnam classics like Apocalypse Now or Platoon, the point is never just to reference those great works -- there's always something else going on in those scenes to make them funny, so the homages simply add another layer of laughter. The care that went into the art direction, for instance (especially in the movie memorabilia on display in an agent's office), will bring a smile to anyone paying attention. Nobody can be faulted for missing some of these subtle pleasures, however, because the big jokes are so consistently uproarious. Everyone from Steve Coogan, as the befuddled British director, to Danny McBride, as a gung-ho special-effects man, to Matthew McConaughey, playing a loyal, unctuous agent, takes full advantage of the numerous opportunities to score laughs. Jay Baruchel deserves particular praise for playing the straight man flawlessly against each and every one of these raging lunatics. But it's Robert Downey Jr. as Kirk Lazarus, an Australian critical darling revered for his chameleon-like method acting, who will keep viewers doubled up with laughter. The character undergoes a radical surgical process that turns his skin black so that he can play an African-American role, and Lazarus refuses to break character, even when the cameras are off. His ongoing verbal battles with entrepreneurial rapper and fellow cast member Alpa Chino (a rock-solid Brandon T. Jackson, whose character gives the film its screamingly funny first scene) become so comically convoluted that they defuse the racial tension. That comedic shock value adds yet another dimension to a movie that already draws upon the rich tradition of Hollywood self-mockery, from Sullivan's Travels to The Player. Time will tell if this film ends up in the pantheon with those poison pen letters to Tinsletown, but it is safe to say that Tropic Thunder is the most consistently funny movie Hollywood managed to produce in the summer of 2008.