In the movie, Minka Kelly plays a college student who befriends her new roomie, played by Leighton Meester.
Meester's character becomes obsessed with her new best friend and things turn ugly.
"It was a really fun ride to watch her play this character," Kelly said. "She creeped me out the entire time."
"I love scary movies," Meester said. "I think it's really weird that I'm the scary person in this movie.
"I'm sane in my life, mostly, nine out of 10, but to just completely let all of that go was uncomfortable, but interesting."
"The Roommate" is rated PG-13 and will be in theatres Friday.