The protagonist enters a murderer’s house with them before they have realised that they are a murderer. Whilst in the house, the murderer leaves to do something and the protagonist starts looking around their house. They find something that indicates to them that the other person is a murderer but have to hide it from the murderer as they return and then try to leave the house without the murderer realising that they uncovered their secret.
The Girl with the Dragon Tattoo (2011)?
Very similar to the opening of….
http://www.imdb.com/title/tt0289765/?ref_=rvi_tt