The former Friends star filmed The Bounty Hunter opposite Butler in New York last year and the pair are currently celebrating Aniston's upcoming 41st birthday in Mexico with friends.
Speaking to Access Hollywood's Billy Bush, she said: "We just had so much fun together you know. He's just fun. Gerry's the most unpretentious guy.
"He's a guys' guy, but he’s absolutely the most lovely and sort of self-deprecating and just what you see is what you get. We look at work the same way… we always had each other's back."
Other friends who have jetted to Cabo San Lucas for the birthday celebrations reportedly include Courteney Cox, Jason Bateman and Sheryl Crow.
Aniston's multi-part interview begins airing on Access Hollywood tonight.
> Aniston dating co-star Gerard Butler?