No Good Deed (2002)

In searching of a runaway teenager, cello-playing cop Jack Friar stumbles upon a bizarre band of criminals about to pull off a bank robbery. While the criminals try to decide what to do with him, Jack is left alone with the leader's beautiful girlfriend, Erin who is a master manipulator of the men in the gang. Jack teaches her to play the cello and the two share a mutal attraction. Jack can't figure out if she's for real, or manipulating him, too.