While he lays on the stereotype of the comical Irishman, always ready with a joke and a laugh, to cover over his reactions to the things he observes, there are times when the veneer cracks. Sometimes it's nothing more than him having to stare into a mirror to make sure his mask in place, other times it's a certain deadness in his eyes and, when he's off duty, he'll go on drunken binges in order to dull his pain. By the time the fifth season comes around he's barely able to hold it together. Two fellow undercover officers, he had been their contact on the "outside - their cover officer, go missing on the job. The guilt and anxiety he feels over their disappearance manifests itself in anger and frustration at his fellow officers, the amount he drives himself to find them and the depth of his emotional reactions as he uncovers some of the secrets behind their vanishing.
While Nesbitt is wonderful, the rest of the actors are equally talented. While episodes during the first two seasons each presented a complete investigation, which didn't allow for much character development among the criminals, the final three seasons were each made up of multi-part episodes dealing with a single case. In each we become as close to the criminals as Murphy does. While there are those who you're not going to spare any sympathy for, Series Five in particular deals with smuggling people in from Eastern Europe and trafficking young women as prostitutes, there are also times when even when we don't like the person in question we learn enough about them to understand how they ended up being the people they became.
Series Four in particular is wonderful for this as we are introduced to two brothers. Both former Loyalists - Protestant Irish who had carried out terror attacks against Catholics - Drew, (Liam Cunningham, and Billy Johnstone, Brian McCardle, have left Belfast Ireland and moved to England upon being released from jail. As they had both been violent criminals into drugs, robbery and assault before they were jailed, when violence starts to erupt in the housing estate they now live in they are immediately suspected. However, the elder brother Drew claims to have converted to Muslim in jail and renounced the ways of violence and crime.