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When I found out that it had aliens in it, I wasn't overjoyed," David Duchovny says, lounging in an orange contamination suit during a break from the shooting of Evolution. "Whatever I do, people are going to compare it to The X-Files." Ultimately, he took the job "because the script was funny, and I wanted to experience what it was like to do a big Hollywood comedy blockbuster."

What the actor has discovered, for one thing, is that making a big-budget, special-effects comedy - a genre that the film's director, Ivan Reitman, helped create with Ghostbusters, in 1984 - requires an extraordinary imagination. In Evolution, which was cofinanced by DreamWorks and Sony, Duchovny and Orlando Jones play a pair of hapless community-college professors who tussle with both government officials (including Julianne Moore) and extraterrestrial forces after stumbling upon a bizarre discovery: a meteor teeming with pulsating alien spores that are capable of condensing 200 million years of Earth's evolution into about 30 days, with twisted results. The various alien spiders, flies, and walking logs that Duchovny and Jones encounter will be added in postproduction by Oscar-winning visual-effects supervisor Phil Tippett (Jurassic Park). "It's a real challenge," says Duchovny of working opposite invisible costars. "You don't want to keep relying on stock reactions."

Today, Duchovny is shooting a scene in which he and Jones reenter the cavern where the meteor has settled and see a small creature wander to the foot of an alien tree, which has octopuslike tentacles. The various otherworldly flora and fauna that fill the Manhattan Beach soundstage were created out of polyurethane and found objects (dog toys, Christmas ornaments), but since the animated creatures must remain imaginary, Reitman's voice will have to suffice for now. "Uh-oh," the director says into a microphone that's connected to earpieces hidden inside the actors' contamination suits. "[The creature] is nearing the tendrils. The tendrils go down. They roll it up, up, up, and...chomp!"

Jones's eyes widen. He waits a beat. "The tree just ate it," he deadpans to Duchovny. After five takes, Reitman seems satsified and calls for lunch. "Okay, everybody," yells the first assistant director. "After lunch, we shoot the scene where the walking log steps into the vermicelli tree and gets eaten."