When Jennifer Aniston first met Justin Theroux on the set of Tropic Thunder in 2007, he didn't exactly leave the best impression.

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"I met Justin about five years ago and I thought he was very sweet and he was always very nice," Aniston, 43, tells Pop Sugar. "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."

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Aniston and Theroux, 40, fell in love while shooting Wanderlust in the fall of 2010, but the actress "didn't recognize him at the camera test." (Theroux's character has a bushy beard and long hair in the film; he's since shaved and cut his hair.)

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"That's what's so great about him," Aniston raves of Theroux. "He can do different looks and he just disappears."

Wanderlust, which also stars Paul Rudd and Malin Akerman, hits theaters February 24.

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