Aniston told People magazine she was "impressed" with the ring Theroux picked out for her.
"I had no idea, and I had no idea what I wanted. He just knows what I like," she said.
Theroux "worked with the girls that I have worked with for years and years and years, so he kind of was picking the specialists' brains," Aniston said. "He kept it within a very small circle of people. It was one of the sneakiest jobs I've ever seen, actually.
"It's hard to pull one over on me," she added, "where I'm not like, 'Hmm, you're acting really strange.' It was ... a very covert operation."