Brian Cox plays Big John which is likely the aspect of the film that caused it to be rated NC-17, as Big John is the pedophile who slowly becomes interested in Howie after Howie and his friends stole from him. Cox initially is full on creep, the very definition of it. Unlike say Stanley Tucci in The Lovely Bones, Cox plays the part very realistically and is very unnerving because of that. He does strive away from this man's method instead being quite blunt about it as he portraying Big John as the man who seems so nice as he approaches the boy with his big smile, and gentle charm yet with those very unpleasant thoughts clearly going around in his mind as well.
Cox doesn't hold back in his portrayal of Big John the pedophile. He does not to try to make him extra colorful, or even say a villain making actually far more disturbing in a way in that he is the day to day pedophile. Cox is so unbearably natural as he both threatens as well as comes onto to Howie at the same time. Cox is all too effective really because he doesn't hesitate, and there is just a great deal of casualness in his performance. This seems all standard for Big John, its what he does and Cox doesn't show him to have second thoughts when he is doing his come on. He makes it clear that this is not something new for Big John rather something he does all the time.
Big John is not a man like Stuart Whitman in The Mark who knows he has some serious problems and can barely live with it. Cox portrays Big John as a guy who lives his actions just fine, there is some regret that Cox brings to Big John to show him as a human being, but Cox makes it abundantly clear that his shame never is enough to rid him of his sunny demeanor. Cox never takes an easy route with his performance making a very strange yet always believable portrait of Big John who most definitely is a creep, but at the same time not as some sort of pure evil psychopath either. He never cops out in terms of the filth of the man's mind yet still seems natural whenever Big John is charming or seems to honestly care about Howie and not in a sexual fashion.
Brian Cox gives a compelling turn here that is easily the highlight of the film. He mixes different elements of what could be entirely separate characters, but he is able to meld them into one single man. Cox is able to fully be the creepy pedophile never shying away by how intense he will be at times or just how unabashed he will be about the whole matter. In the same man Cox does show a man though and he is honestly does bring charm into his performance, and even seems natural in the moments where Big John acts as much more of a mentor. Cox manages to make it so neither part of his portrayal compromises the other succeeding in realizing Big John as a complex character who easily is the most intriguing as well as sickening aspect of the film.