This movieland farce, Ben Stiller’s first writing and directing project since Zoolander (2001), gets off to a rousing start with fake coming attractions for a horrid fat-suit-and-flatulence comedy, a mawkish Oscar-bait drama about a mentally disabled man (“Simple Jack”), and a Brokeback Mountain knockoff set in a medieval monastery (in a waist-level close-up, one monk suggestively fingers the other’s beaded belt). The rest of the movie, in which a small crew of spoiled actors and inept filmmakers struggle to shoot a Vietnam epic in the jungle, never lives up to the hilarity of the opening, partly because the large-scale production smothers the gags but mostly because those gags are so easily smothered. With Stiller, Jack Black, Robert Downey Jr., Steve Coogan, and Nick Nolte, all of them upstaged by Tom Cruise in a spellbindingly grotesque cameo as a bald, pudgy studio executive.