Cast member Jennifer Aniston poses at the premiere of "Wanderlust" at the Mann Village theatre in Los Angeles, California February 16, 2012.[Photo/Agencies]

Jennifer Aniston thought Justin Theroux was "like a serial killer" when they first met.

The 43-year-old actress - who has been dating her 'Wanderlust' co-star for just over a year - thought she could see a "dark" side to the actor but quickly learned he is the "nicest person in the world".

Speaking about their first encounter, which took place on the set of 2007 movie 'Wanderlust', she said: "I met Justin about five years ago and I thought he was very sweet and he was always very nice.

"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."

Jennifer admitted she "didn't recognise" Justin when they met again to work on 'Wanderlust' as he had a bushy beard and long hair for the role, but she insists that is part of the 40-year-old star's appeal.

She added n an interview with Pop Sugar: "That's what's so great about him. He can do different looks and he just disappears."