Jennifer Aniston stepped out Monday for the first time since her engagement to Justin Theroux, but the former "Friends" star was playing coy with the paparazzi - hiding her hand and presumably the new diamond ring around her finger.

Photographers caught the 43-year-old actress on a movie set in Wilmington, N.C., but she kept her hand carefully hidden under a white towel, leaving fans and admirers to keep wondering what kind of rock she's wearing from her new fiancé.

Aniston was on set filming "We're The Millers," a comedy set to be released in 2013, where the usually sweet starlet takes an unexpected turn - playing a prostitute. Her character is hired by an ex-drug dealer, played by her "Horrible Bosses" co-star Jason Sudeikis, who hires her to pose as his wife while moving a shipment of marijuana to Mexico.

Aniston and Theroux, an actor and screenwriter, have been together for 15 months. According to the Chicago Sun-Times, Theroux chose now as the perfect time to pop the question largely because he wanted to wait long enough after Aniston's ex Brad Pitt proposed to Angelina Jolie - but not too long, which could have caused the engagement to be overshadowed by a Pitt-Jolie wedding.

The couple first met on the set of the comedy hit "Tropic Thunder," which Theroux wrote. Aniston dished on their introduction to PopSugar: "I thought he was very sweet and he was always very nice. But I remember thinking he was very dark. At first you think he could be like a serial killer, but he is actually the nicest person in the world."

A friend of Aniston's told The Sun-Times that the nuptials aren't scheduled yet, but any details will be kept tightly under wraps.

"It will be run just like a top-secret government operation," the source said. "I'm betting it will also happen somewhere overseas - where and when you least expect it!"

Click here to see the pictures of Aniston on the "We're The Millers" set.