On a temperate, late spring afternoon, I am wandering through the Warner Bros. lot in Burbank, California, in search of Mixing Stage 3, where I have an appointment with Ben Stiller, who is putting the finishing touches on his new movie, Tropic Thunder, and I'm thinking, Man, I hope this guy doesn't turn out to be a ding-dong.

Here's the thing: I've just come from a screening of his film — a big-budget action-comedy starring Stiller, Robert Downey Jr., and Jack Black as prima-donna Hollywood stars shooting a Vietnam War blockbuster that turns hideously real — and, for me, it edges out Superbad for Funniest Movie Since Borat. It's also well made and handsomely filmed, plus a lot of stuff gets blown up. And Downey — Jesus, is there anything the guy can't do? So, what's my beef with Stiller? Look, I know a thing or two about celebrities, and anyone as rich and famous as Stiller has got to be an arrogant, insecure phony — much, ironically, like the one he plays in Tropic Thunder — who spends all morning squeezing himself into his girdle, rouging his cheeks, and combing his gorgeous locks just so he can strut around like a peacock, throwing cell phones at assistants and making paid friends tell him that he could pass for 22, the crumb.

Such is my state of mind, as my eyes slowly adjust to the gloom of Mixing Stage 3 and I spot Stiller perched Hindu-style on a leather chair in jeans and a navy T-shirt, calmly surveying banks of digital consoles manned by middle-aged tech guys ready to do his bidding. He calls to mind a sullen 13-year-old whose parents gave him a Hollywood studio as a bar mitzvah gift.

Seen on a video monitor the size of a movie screen, Stiller is playing Tugg Speedman, a Hollywood action star hoping to bounce back from his disastrous performance as a retarded farmhand in a film called Simple Jack by playing a war hero in a film called Tropic Thunder. Caught in a hail of enemy fire, Speedman falls to his knees in a slo-mo homage to Willem Dafoe's iconic, seemingly endless last stand in Platoon — his massively chiseled arms outstretched crucifixion-style, bullet-riddled body jerking like a marionette. A sergeant from his platoon, played by an African-American actor you can't quite place (turns out, as you may have heard, it's Robert Downey Jr.), runs a gauntlet of shrapnel and tracer fire to rescue him, mowing down VC guerrillas with his machine gun while shouting "Want some? Get some!" as a chopper spins out of control and crashes just inches away, exploding in a fireball.

The playback stops abruptly, and a sound technician turns to Stiller: "Did you hear how we took out the — "

"Yeah," Stiller says quickly. "I like it. But when it hits, how about more of a real ka-chung — almost like, you know, a cartoon character getting hit with an anvil?" As the sound guy does his thing, Stiller stands up to greet me with a plausibly sincere smile. He is roughly my height, though his upper body appears to be marginally more developed than mine. He seems to have a larger-than-average head, a prerequisite for screen stardom. His good bone structure and strong features are those of a leading man, with a touch of nebbish about the mouth. He explains that his assistant isn't around and offers to fetch me a Diet Coke. This must be some kind of trick. I indulge him anyway.