Big budget, big stars, big concept – surprisingly little payoff. Tropic Thunder delivers extravagant set pieces and a raft of superstars, yet somewhere along the line, director/co-writer/star Ben Stiller forgot to concoct material that might elicit more than a mild chuckle. To an even greater extent than Zoolander, Stiller’s latest double-duty effort in front of and behind the camera relies, first and foremost, on its concept to generate comedy, a strategy that – because actual jokes are pretty sparse, and feeble when they do appear – leaves one feeling like the proceedings should be funnier than they are. The film’s story is an Apocalypse Now riff in which three idiot stars – action-hero doofus Tugg Speedman (Stiller), junkie comedian Jeff Portnoy (Jack Black), and Australian Oscar-winner Kirk Lazarus (Robert Downey Jr.) – go off into the jungle to make a Vietnam epic, and wind up in the midst of a genuine military conflict that they believe is part of the script. This leads to a host of tepid scenarios featuring actors yelling cut at real-life soldiers, Tugg’s agent (Matthew McConaughey) trying to get his client a TiVo, and a studio exec (Tom Cruise) cursing like a sailor at everyone in sight. All of these characters and situations are meant to spoof various aspects of the Hollywood machine (thesps’ vanity, studio boss’ crazed megalomania, etc.), though the only bits that have some sting involve Lazarus, a Caucasian who’s undergone a skin-pigmentation procedure to play an African-American, and finds himself perpetually stuck in stereotypical character. Embracing his moronic blackface routine with such goofy relish that the joke is acutely, amusingly on himself, Downey Jr. is pretty marvelous. Everyone else, however, is wasted in dreary one-note roles (Stiller is the vapid idiot, Black craves heroin), and even when its satire is spot on, Tropic Thunder fizzles simply on account of a dearth of honest-to-goodness humor.