Kill 'em with kindness!

When Jennifer Aniston, 41, met sexy Just Go With It costar Brooklyn Decker, 23, she was all smiles.

"The first time we met, she came up to me and gave me a big hug," the Sports Illustrated swimsuit model tells Entertainment Tonight. "She's been like that the whole way. Someone in her position doesn't have to be that way and she is."

A set source told Us Weekly that Aniston is "intimidated" by her younger costar. "She's very conscious of her body...She's obsessed with staying fit and young."

Toward that end, the 5-foot-6 actress "rarely misses a workout" no matter where she is, the source says. In between scenes for Just Go With It (in which she flaunts her bod in a hot-pink bikini), the 5-foot-6 single star hit the gym for at least an hour daily, sipped pureed papaya, pineapple and mango and dined on veggies and fish.

Looks like the hard work paid off. "She's stunning," Decker gushed. "She's such a great mix of beautiful and kind."

Adds Decker: "The most surprising and amazing thing about her is she treats everyone like she's been best friends with them for years."