The build up to the confrontation between both men is something that brings a sense of climax and intensity to the development of plot. I think that Jimmy confronts Dave with the understanding that he in fact killed Katie. This is something that has been lingering in his mind for some time, and confirmed with Celeste tells him as much. As Jimmy is a powerful figure, and one that believes in traditional notions of justice almost as much as consolidation of his own power, he confronts Dave with Val and in a setting where Dave is powerless. Jimmy works the emotional climate of suspicion and tension to the point where Dave vomits out of discomfort, giving Jimmy and Val the opportunity to confront him outside. In doing so, Dave reveals himself to be chained to the demons of his past, not knowing what to do in a critical situation. He discloses his killing of the child molester than night, but also lies to Jimmy in the hopes that it will allow him to live. The entire confrontation is one of miscommunication. Dave has never been able to communicate with anyone what happened to him as a child, while Jimmy has never been able to communicate in the role of a true friend as to what happened to Jimmy. Celeste's information ends up dooming her husband, something that is rooted out of her own miscommunication with her husband about where he was the night of Katie's murder and what happened to him as a child. The miscommunicaiton that Dave and Jimmy have experienced between one another is brought out in the confrontation where there is only one evident conclusion where more will be given into the Mystic River of both Boston and the interpersonal connection of those who live there.