"He's a little edgier than the guys she's been out with, but they're having a really good time," People magazine reporter Lesley Messer told ABC News Now's "What's the Buzz." "Apparently, she's really happy."
Although they've been dating a few weeks, Aniston threw a small get-together May 27 to introduce Theroux to her best pals. Chelsea Handler and Courteney Cox, putting to bed rumors that she and Cox were on the outs.
There has been "tension" between the former "Friends" co-stars over Cox's separation from husband David Arquette, according to a report in this week's Us Weekly magazine.
Us quoted an insider saying, "Jen thinks Courteney is being callous" by spending time with her "Cougartown" co-stars Brian Van Holt and Josh Hopkins, without formally filing for divorce from Arquette.
Aniston's rep called the report "completely false," telling the Los Angeles Times, "There was no falling out. Jennifer has always and continues to consider Courteney a dear and close friend."
Cox apparently approved of Aniston's new beau.
"They all had a great time and enjoyed Justin's company," a source told People.
Aniston, 42, and Theroux, 39, first became friendly on the set of "Wanderlust" in the fall, according to the magazine. Aniston has been known to fall for her co-stars. Remember Vince Vaughn? Gerard Butler?
But Theroux is different. For one thing, he doesn't have those dashing leading-man looks. He's darker and scruffier and more likely to be cast as the villain: He was Drew Barrymore's evil ex in "Charlie's Angels Full Throttle" and the evil wizard Leezar in this year's "Your Highness."
He can play drama or comedy, big or small screen, and he has serious writing cred. He co-wrote the comedy hit "Tropic Thunder" and, on the recommendation of star Robert Downey Jr., wrote the screenplay for "Iron Man 2."
In her new movie, "Horrible Bosses," the queen of romantic comedies will be shedding her girl-next-door image to play a "sexually aggressive" dentist who harasses her male assistant, played by Charlie Day.
Aniston reportedly has filmed one topless scene, a first for her at age 42. But whether the scene will remain in the film when it's released June 8 has yet to be decided.
That's not all. Later this fall, Aniston could also appear topless in Judd Apatow's "Wanderlust," in which Aniston and Paul Rudd play a couple who drop out of the rat race to become hippies. Aniston not only rips off her shirt but smokes pot and has a threesome, according to HollywoodLife.com.
She also has out a new ad for SmartWater – this time she's posing next to a bottle of the designer water, without any clothes.
Aniston has shown she's not afraid to show some skin to promote a product (her fragrance line last year) or herself, for that matter (the cover of GQ in January 2009).