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What's Justin Theroux really like?

"He's just the most cold, icy, fishy kind of a guy," his girlfriend Jennifer Aniston joked on Good Morning America Monday. "I'm trying to warm him up, but you know..."

All kidding aside, Aniston, 43, said her beau of less than a year "is really cute" and "a great actor." (The two became an item while shooting Wanderlust, in theaters February 24.)

Aniston added that she's "known Justin for a long time. We met when he was shooting Tropic Thunder."

Shooting Wanderlust in a remote location with Theroux, 40, Paul Rudd, 42, and Malin Akerman, 33, was a dream come true for Aniston.

"The main thing that was so refreshing for me was being in Clarksville, Georgia, which was so far in the middle of nowhere. There's sort of a parallel experience happened between what happened with my character in the movie and with me in life," Aniston said. "All of my guards fell away. I felt this weight off of my shoulders of constantly being on guard or looking over my shoulder or [asking], 'What's that over there?' or 'Who has a cell phone?'"