When an eclectic collection of actors assembled to film a dark thriller about carnivorous monsters stalking human prey, they knew they'd have to endure intense training, long hours, elaborate stunts and damp, claustrophobic sets. What they didn't realize, however, was that their greatest acting challenge could be summed up in one word: Brian.

"Brian was the guy," laughed Eddie Cibrian, the dark-haired, square-jawed actor who portrays cave-diving adventurer Tyler. "The guy who got into the costume."

Actress Lena Headey, who plays a fellow explorer named Katherine, shared in the memory: "He's the guy, the creature guy."

As is the case with most modern movie creatures, when the "Cave" actors were filming the scenes that called for them to flee from the winged beasts down below, they were actually running from a stunt actor wearing an enormous monster head. After such films are finished shooting, filmmakers use computer graphics to enhance the beast, whose physical presence on the set is mostly so the actors' sightlines will be in the right place. For Cibrian, Headey and their co-stars, the most difficult part of the shoot was trying to convey fear while tears of laughter were frequently forming in the corners of their eyes.

"He'd knock over bits of the set because he couldn't really see," giggled Headey. "He had the 8-pound head on."

"He could see through his neck," Cibrian remembered, raising his hand high, "but the head was way up here."

"He'd lift it up to look at you and talk to you," Headey said as the two doubled over in laughter. "He was like a scary horse."

Luckily, the two relatively inexperienced film actors had Cole Hauser and Morris Chestnut by their side, genre veterans who could dispense valuable advice on how to properly run from big, scary things.

"This thing would probably kill the anaconda," insisted Chestnut, who was devoured in one giant gulp during last year's "Anacondas: The Hunt for the Blood Orchid." "They'd have to battle it out on their own," he said of a hypothetical battle between himself, the enormous snake and the "Cave" beasts: "I've got to get out, man."

The 30-year-old Hauser, meanwhile, is perhaps best remembered for taking on CGI monsters in "Pitch Black," alongside an equally monstrous Vin Diesel. "This is obviously a little different in the sense that 'Pitch Black' happens on another planet," compared Hauser. "[This] is a whole different medium than I've seen in 'Alien,' 'Pitch Black,' you name all the sci-fi films that have come out; this is a whole different thing. It's on our own planet, so you can relate to some of what's going on, and I think the smart thing about the movie is that it relates to the average person."

Yes, even someone like Brian Steele -- buried deep within that huge, clunky head -- could see that what separates "Cave" from its predecessors is a heightened sense of plausibility. Romania does indeed contain some 12,000 caves and is suspected to have just as many more that remain uncharted. Micro-ecologies have been discovered in many of the caves, most notably during a 1980s expedition that found dozens of unknown invertebrate creatures in a cave near the Black Sea.

"We know more about outer space than we know about our Earth," Hauser reasoned. "Talking to the real people, the cave divers who do a lot in the West Isles, they truly are Earth's astronauts. They're out there exploring Earth and trying to find new places that nobody's ever seen before, and it's pretty amazing. What they've done in the last 10 years is groundbreaking."

Appropriately enough, first-time director Bruce Hunt (graduating to the big chair after working second-unit on the "Matrix" films) insisted that the actors learn to use the equipment worn by divers like Dr. Christi Lascu, the real-life explorer who helped uncover those Black Sea creatures and served as a consultant for the film.

"It was very intensive," remembered Cibrian. "We trained for about six weeks in L.A., rock climbing and scuba training. ...We scuba trained for about three days and then went right into these things called rebreathers. It's technology that they actually use in space, the technology that the Navy SEALs use. It's not an open-circuit system, it's a closed circuit, so you basically rebreathe your own air; no bubbles, it just scrubs [the air] out. Two little tanks and you can stay underwater for like 20 hours."

In all the world, there are only about a dozen people fully certified in the new technology. "Very complicated to use," Cibrian remembered. "It would take you, I think they said, six to seven years to be fully certified to use these things by yourself and own and operate one."

For Headey, the rebreathers offered a once-in-a-lifetime opportunity to peek into the lives of divers, as well as the peaceful existence of aquatic creatures. "There's no bubbles," she marveled. "It's really peaceful ... kind of Zen."

While the training, equipment and creatures in "The Cave" may be closer to reality than most science-fiction films, Hunt and his writers admit they were happy to wield their dramatic licenses when necessary. The most obvious example is the infection spread by the beasts when they attack, which threatens to turn the divers into winged creatures themselves while setting up multiple opportunities for sequel storylines.

"We'll see what happens," Cibrian commented on his participation in another "Cave" movie.

"Yeah," Hauser volunteered, saying he'd love to return. "But first we've got to make some money and promote this thing the right way. ... I'm thinking about [Friday] the 26th and opening this thing up when it's strong, and we'll go from there."

Headey, for her part, said she'd love to do a sequel: "Absolutely, in a heartbeat," and she joked that she'd even be willing to put on the giant goofy monster head herself next time. "I can't wait," she smiled, "I was jealous the whole way through."

The prey offering to take away the job of its predator? It sounds like Brian might want to start polishing up his résumé.

Check out everything we've got on "The Cave."

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