The actor jetted to the Tahitian island with fiancée Jennifer Aniston on Aug. 7 for a week-long stay in paradise that also marked two years since the couple became engaged.
"It [was] a romantic trip to celebrate their engagement anniversary and Justin's birthday," says an insider. "It's their first vacation together in Bora Bora and they were excited about the trip."
It was also a nice break from work – Theroux, 43, has been filming his HBO series, The Leftovers (which was renewed for a second season Tuesday), in New York City, while Aniston, 45, has been busy promoting Aveeno and her haircare line, Living Proof.
During a July 28 event at stylist Chris McMillan's hair salon in L.A., Theroux surprised Aniston by popping in to steal a kiss.
"Jen had no idea that he was stopping by and she squealed with excitement when she saw him," says a source. "They genuinely looked very happy together. They kissed hello and Jen almost got flustered. You could tell that she really liked having Justin there. Justin kept smiling at her excitement. He was very sweet to her."
While wedding plans are still in the works, Aniston has a heavy workload in the coming weeks. At the end of the month she will be doing press for the Elmore Leonard adaptation Life of Crime. In September, she is scheduled to attend the Toronto Film Festival, where her film, Cake, will premiere, while her Peter Bogdanovich film, She's Funny That Way, which re-teams her with her Marley & Me costar Owen Wilson, will debut in Venice.
Yet regardless of workloads or distance, the couple continues to make an effort to stay close, says the insider. "Despite their long engagement, they do seem very happy together," the source adds. "Jen is just happy to be sharing her life with Justin."