The former Friends star and Theroux became engaged in August, after dating for over a year.
Aniston told People that she was impressed by Theroux's ring choice, an eight-carat diamond.
She said: "I was impressed. I had no idea, and I had no idea what I wanted. He just knows what I like."
The 43-year-old was equally impressed by Theroux's proposal, which she said she hadn't seen coming.
"It's hard to pull one over on me," Aniston added. "Where I'm not like, 'Hmm, you're acting really strange'. It was... a very covert operation."
Aniston and Theroux reportedly may be planning a three-day wedding celebration, though they haven't specified a date.