"We just had so much fun [making the movie] together, you know. He's just fun. Gerry's the most unpretentious guy," Aniston tells Access Hollywood in a multi-part interview set to begin airing Tuesday night.
"He's a guy's guy," she adds, "but he's absolutely the most lovely and sort of self-deprecating [person] and just what you see is what you get. We look at work the same way … We always had each other's back."
Butler, 40, was among the large coterie of friends who joined Aniston for her 41st birthday celebration in Los Cabos, Mexico, this past week. The pair were also nearly inseparable during the Golden Globes weekend in January.
By all accounts, the long birthday weekend was a blast, and Aniston says there was just one rule – no one could say the number 41 out loud. "Thirty was a hump, but you know 31 is fabulous," she jokes. "It really does get better, doesn't it?"
Why did she choose Mexico where she has gone many times before? "These people survive on us coming down and spending money," Aniston explains. Considering the country's troubles with swine flu and drug trafficking, "It sort of made sense to sort of say, 'Hey, let's help out Mexico.' "