Men overboard! In the new movie The Guardian, Kevin Costner plays a grizzled instructor of Coast Guard rescue swimmers—if your cruise ship hits an iceberg, page them—and Ashton Kutcher is his cocky recruit. While training with a real rescue team, Costner, 51, and Kutcher, 28, bonded like brothers (wait, they are brothers—both action men pledged Delta Chi in college).

The dynamic duo took time to chat about workouts, pig-outs and pajama parties with Mrs. Kutcher—Ashton and Demi Moore celebrated their first anniversary on Sept. 24—with PEOPLE's Darla Atlas.

So, what was it like working together?

KUTCHER
I made a friend along the way, which is always a pain.
COSTNER
The Christmas card list grows and grows and grows. "Oh my God, I've gotta give him a ham too?"

Who's the better swimmer?

COSTNER
Ashton's stronger. He trained with Olympic swimmers, and there were people who wanted to kick his ass. There's a saying in the business: "Don't give me the fast guy, give me the mule." Give me the guy who's strong and isn't going to quit. That's mulehead over there.
KUTCHER
Now the article will read, "Kevin says, 'Ashton is a mulehead.'" Is that far from a jackass? I don't know ...

What surprised you most about your costar?

KUTCHER
Kevin likes banana splits at midnight. One night we got this craving for banana splits, so we went to the grocery store. Can you imagine the two of us and our wives running around the grocery store, pulling down ingredients for banana splits?

Your families came to visit you on set in Louisiana?

KUTCHER
Yep.
COSTNER
Demi and Ashton didn't stand on a lot of ceremony. One night in the hotel, me and Chris [Baumgartner, his wife] called and said, "Do you want to come over?" And they were in their pajamas; they just came in like that. It's nice when it doesn't have to be, "Oh, we can't come because blah blah blah." It's like any relationship you want to have: There's no BS.

Ashton, ever try to punk Kevin while you were filming?

KUTCHER
No. We had to make a truce. When you're working on something, you have to build trust with the person you're working with.
COSTNER
I begged him, "Pleeeease don't!" And he was true, you know? That meant a lot; some people can't get away from their own nature, and Ashton can.
KUTCHER
Actually, one day on the set, we'd been swimming for eight hours, it was hot, and Kevin was sitting in a chair out in the sun with his sunglasses on and his head back, sleeping. I took this big towel and wrote "Action Hero" on it, with a big arrow. I got right behind him and held it up. I have a really good picture of it, but no one will ever see that. It's just for me.

Kevin, how does Ashton shape up as an action hero?

COSTNER
Everyone in his position faces [skepticism]. I did for years. Hello, he didn't just land here from Mars. If people think it's so easy to be a star on a television show, go try it yourself.
KUTCHER
[grins] I like you. You're nice.

So, did you two tough guys hit the gym together?

KUTCHER
We worked out together the first day we were filming in Louisiana.
COSTNER
We had to make sure we didn't puke in front of the rest of the guys. We didn't want to be like little wussies.
KUTCHER
The key is, don't eat too much before you work out. That's the tip.

TO THE RESCUE: REAL GUARDIANS

Each year approximately 75 recruits sign up with the Coast Guard Rescue Swimmers, the elite squad of men and women featured in The Guardian. Their job: to jump from helicopters into stormy seas and violent floodwaters to save those in harm's way. But in the end, only half the recruits survive the grueling 18-week training at the Coast Guard's Elizabeth City, N.C., base. "He could be the biggest guy who seems so strong, and I'll think, 'Oh, he'll make it,'" says First Class Petty Officer John Hall, an instructor since 2002, who plays himself in the film. "The next day he's washed out." Turns out the ability to do one-armed push-ups isn't enough; hopefuls must show the mental toughness required to survive the harrowing life-and-death situations that are just another day at the office (including rescuing "drowning" officers who are physically uncooperative). "At some point you're going to be tested," says Mark Wamble, 30, a rescue swimmer since 1997. "Your mind's going to say no, and you'll feel like you can't do it."

Created after a 1983 storm claimed 31 crew members from an overturned cargo ship in the Atlantic (the Coast Guard realized its existing techniques and equipment were inadequate for such extreme circumstances), the group now springs into action during such dangerous maritime conditions as last year's Hurricane Katrina. Rescue swimmers scoured the devastated Gulf Coast in the thick of the storm and were credited with rescuing an estimated 33,520 people. But Hall and his charges, such as Joshua Mitcheltree, who survived his grueling training class in '02, shrug off words like hero. "I try to tell myself I'm not," says Mitcheltree, 24, who has a walk-on role in The Guardian. "Otherwise I'll float away with a big head." Adds Hall: "I don't do this for the glory."