Nick was killed before his release in the jail cafeteria, and so Nick took his identity so that he could meet up with the infamous Ashley he kept hearing about, forming a mental picture of her in his head.
Nick is really a car jacker who overheard his jail cellmate, Nick, reading his letters from a penpal named Ashley aloud. There is one minor detail, however, that may hinder their plan. Her brother kidnaps Nick, holds a gun to his head and commands him to draw up a sketch of the security points in the casino, since he used to work there and knows all there is to know about the casino. However, her brother (Gary Sinise) wants Nick to help them pull a heist on Christmas Eve at a local Indian casino, which is run by a money hungry wannabe (Dennis Farina). Upon his release, a long-time pen pal named Ashley (Charlize Theron) greets him and they immediately hit the sack. 'Okay, it's about this guy, who's not who he says he is, who everybody thinks he is, who fools these guys who thinks he is, falls in love with this girl who thinks he is, even though he isn't, and they try to pull a heist with his expertise, even though he has none.' Nick (Ben Affleck) has just been released from jail for defending his girlfriend in a fight prior to being thrown into the slammer.
I can imagine the pitch for 'Reindeer Games' being thrown at the execs behind the picture.