Sandra Bullock and Ryan Reynolds have already seen each other naked -- filming a hilarious scene for their blockbuster 2009 comedy The Proposal.

"I don't know how it would have been with anybody else," Bullock, 46, has said of shooting in the buff with her sexy pal.

Indeed, as Bullock and Reynolds, 34, grow closer in the wake of their shocking marital splits --  she was "glowing" during a flirty New Year's Eve with him in Austin, Tex. -- it's a culmination of a long, close friendship, as explained in the new Us Weekly.

How did Sandy meet Ryan, anyway? Way back in 2000, when Reynolds, then a star of ABC sitcom Two Guys, a Girl and a Pizza Place, was introduced to Bullock through a producing partner. The low-key, funny duo struck up a platonic friendship.

The bond endured even as Bullock dated Ryan Gosling and he romanced fiancee Alanis Morissette; after Bullock wed Jesse James and Reynolds tied the knot with Scarlett Johansson, the foursome even went on vacations together.

"They're so good together," a Proposal set insider tells Us. "They have fantastic chemistry and they're great people."

Delve deeper into their relationship -- how "Sandra's the perfect person" for Reynolds and more on their "hot" New Year's Eve -- pick up the new Us Weekly, out now!