Affleck's brother Casey takes the leading role of Patrick Kenzie who works as an investigator for missing persons along with his girlfriend/partner Angie Gennaro (Michelle Monaghan). Patrick acts as our guide through the case of the missing little girl as he is hired on by the girl's aunt in order to explore all options in their attempt to find her. Patrick seems to be one of the least shady character considering the oddly volatile nature of the police he must work with as well as the girl's mother (Amy Ryan) who is more than a bit of a low life. Patrick is called upon though actually because he is able to try to derive information from the sort of criminal elements in Boston that the cops cannot even hope to approach. Affleck plays the early scenes of his performance well by just showing Patrick as a casual observer of the events. He sees what's going on TV, even makes fun of the cop's efforts, and Affleck is very good at showing that sort of concern you get from hearing a story like this on TV. He's moved by it to be sure as any normal person would be, but he carries that distance since he doesn't know those people.
Affleck uses this as a great starting point for his character as he takes on the case, and at first keeps himself as just doing something that seems routine. Affleck portrays Patrick's demeanor at first as a detective just processing the case for himself in an intelligent fashion. Even when Angie is clearly becoming quickly emotional at the prospect of what could happen to that girl, Affleck keeps as rather calm and collected. After the preliminaries are finished though Patrick takes his unique approach to finding the truth which means visiting more than dank pits. Although Patrick is allowed in these places and even allowed to speak for more than a moment there is always a tense atmosphere as the people he speaks to have much to hide. Affleck has a bit of a challenge in these scenes since he has to be the investigator who gets to the information from these people, even though he's not exactly a Popeye Doyle kinda figure. Although Affleck is obviously not physically imposing, he kinda works with here. In the speaking Affleck keeps Patrick's manner as about cordial as he can be while keeping an underlying intensity as though he might snap and that he's not a man to be pushed around.
Although the film keeps a tight focus on progressing the plot through investigation Affleck does well to realize Patrick as a character, and you understand him even though the film never stops simply to explore something about him. Affleck through seemingly minor moments of the investigation alludes to perhaps a bit of a darker past in Patrick as well through the side found in his his particular method of interrogation both in the way he gets rough, but as well is so comfortable in speaking with known criminals. Affleck shows where Patrick comes from as he wears his way with the town as an essential part of him. In addition to finding the character with the investigation it is Affleck who often brings the most dramatic weight to it. Affleck does begin with that distance in much of the early investigation, but as the trail becomes murkier this gradually changes. Affleck is particularly good in his scenes with Ryan as she reveals the mother to be a terrible mother but one who still cares about her daughter. Affleck is very effective in the way he delicately grows Patrick's empathy for in turn becoming more emotionally attached to the case.
Affleck makes every progression of the story hit harder through his honest reflection of what it does to Patrick's reserve especially at one point where it appears the case has been lost. Affleck's handles the changes in Patrick particularly well as he shows him as a man becoming hardened by the developments. This leads to one scene where he goes along with the cops to raid the home of known drug addicts and a pedophile. Affleck is terrific in the scene as Patrick takes an extreme action. The action could have seemed too much for Patrick to suddenly to something of this gravity, but Affleck is absolutely convincing in the scene. This is partially in that underlying intensity that Affleck gradually built up in the proceeding scenes, but also he brings out the severity of the scene through his gut wrenching reaction within the moment. Affleck though does not suddenly just make Patrick this hard man though, as that would be against the nature of the character he has created up until this point. Affleck is very good in having that moment where Patrick seems to look back on what he has done, and shows how that this action will haunt him for probably the rest of his life.
The last act of the film reveals a twist, that technically is established as well as it probably could be, but on re-watch it is a bit hard to swallow. Nevertheless it's not a complete failure through moral dilemma that it creates in doing the right thing as normal or at least legal morality dictates, or doing the thing that appears would be best for the person in the center of the question. The decision falls upon Patrick and in turn Affleck's performance is once again essential in bringing out the power of the story. Affleck brings the needed passion as Patrick makes his decision. He gives a complete understanding on his side by showing that Patrick's empathy for the victim, and determination to do what is right is unshakable at this point. Affleck does not keep it simple of just being right, the difficult of it is in his voice and his eyes, but so is the conviction of his beliefs. Of course we see the result of the decision and gives Affleck one final scene in seeing a non too favorable outcome for this decision which changed many lives permanently. Affleck's final reaction is flawless as he presents Patrick as almost in a bit of a daze unable to know for sure that he made the wrong decision, but also unable to be sure he made the right one.