The 43-year-old, who was earlier married to actor Brad Pitt, got engaged in August, reported People magazine.
"I was impressed. I had no idea what I wanted. He just knows what I like. 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.
"He kept it within a very small circle of people. It was one of the sneakiest jobs I've ever seen, actually," she said.
The actress says she was impressed with Theroux's "secret operation."
"It's hard to pull one over on me, where I'm not like, 'Hmm, you're acting really strange.' It was a very covert operation," she added.
