Jennifer Aniston sure knows how to flip the script.
The 42-year-old star fell hard for costar Justin Theroux on the Georgia set of Wanderlust last fall, and nothing was going to get in her way, not even Theroux's 14-year relationship with live-in girlfriend Heidi Bivens.
"Jen went after him," a set source tells the new Us Weekly, out now; as Wanderlust's shoot continued, Aniston and hunky Theroux, 39, began having secret trysts at her temporary digs, a huge, secluded house on Georgia's Lake Rabun.
Charmed by Theroux's quick wit, his edgy downtown hipster style and impressive background as the screenwriter behind Tropic Thunder and other hits, Aniston overlooked her secret beau's relationship status, a Theroux insider says: "Jen wanted Justin and it didn't matter that he was already with someone."
Fast forward to March of 2011 -- when Theroux abruptly ended things with fashion stylist Bivens, 35, who moved out of their NYC apartment in June. Blindsided and hurt, Bivens "didn't know Aniston was the catalyst" behind the split, the source says, "until now."
And, yes, the bitter irony -- Aniston's husband Brad Pitt infamously left her for Angelina Jolie six years ago after falling in love on set -- is lost on no one. "It's amazing she would go for an attached guy after what happened to her."
Adds a Bivens insider: "It sucks when your ex dates the most famous woman in the world so soon after a breakup."
For much more details on Aniston and Theroux's shocking fast-track romance and the fallout -- how Theroux's ex thinks it's "tacky," how he's "obsessed" with becoming a superstar, and more -- pick up the new Us Weekly, out now!