Middle Age Hollywood

After shooting a movie with demanding action scenes, it's tough not to bond with your costar – especially if she's Jennifer Aniston.

"I literally became a member of her family," Gerard Butler told PEOPLE of his Bounty Hunter costar at their film's London premiere Thursday night. Just don't confuse that closeness for something romantic. “I went down for her birthday in Mexico,” Butler said. “Of course, everyone takes that as something else, but we are very good friends.”

And Aniston reciprocates those feelings. She called Butler, "the best!" Stepping out on the red carpet in a strapless, silver Valentino Couture minidress – despite subzero temperatures – she added, "He's funny and handsome – [a] nice combo!"

The two, who recently posed for a rugged, sexy W magazine cover, had to endure some grueling physical challenges while shooting their action comedy, which is due in theaters March 19. Turns out, Butler isn't quite as macho he appears onscreen. "I'm like a baby about that stuff – I'm always whining," he said.

But that wasn't the case with his sexy costar. Though she spends most of the action flick bound and cuffed, Aniston was anything but hapless during the film's more high-octane scenes. "She’s dragging doors behind her, and her handcuffs are cutting away at her wrists," Butler said. "She had to go through a lot of stuff in this movie and she never complained."

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