Special Investigator Dewey is a family man. He has a wife, Maria, and two young sons, Alvin Jr. and Paul. Dewey and Maria appear to have a close, loving relationship. He's described as a "lean and handsome fourth generation Kansan of 47" (2.8).
Dewey had known the Clutters, seeing them weekly at church and visiting their home. This makes the murders personal for him.
"But even if I hadn't known the family and liked them so well, I wouldn't feel any different. Because I've seen some bad things, I sure as hell have. But nothing so vicious as this. However long it takes, it may be the rest of my life, I'm going to know what happened in that house: the why and the who." (2.8)
Dewey's every waking moment is consumed by the case:
His mind automatically rejected problems not concerned with the Clutter case. Marie and many of their friends had begun to wonder at the completeness of his fixation. (2.222)
He knows what will happen to him if he doesn't solve the case ASAP:
"Years from now I'll still be running down tips, and every time there's a murder anywhere in the country even remotely similar, I'll have to horn right in, check, to see if there could be any possible connection. But it isn't only that. The real thing is I've come to feel for Herb and the family better than they ever knew themselves. I'm haunted by them. I guess I always will be. Until I know what happened. (2.222)
The townspeople worry about his health, when they're not haranguing him to find the killers already.
"Why don't you arrest somebody? That's what you're paid for […] If you ever run for sheriff again, just forget my vote, because you ain't gonna get it." (2.140)
Despite his obsession with the murders, we learn that he's starting with what turns out to be a completely wrong premise.
A few conclusions were unshakeable: he believed that the death of Herb Clutter had been the criminals' main objective, the motive being a psychopathic hatred, or possibly a combination of hatred and thievery. […] Attendant upon these beliefs was his conviction that the family had known very well the persons who destroyed them. (2.148)
Dewey's portrayed as honest and conscientious. Even though he's ecstatic when he hears about Floyd Wells's revelations, he makes sure not to get ahead of himself.
"I [Mrs. Dewey] told him, 'Now, Alvin, don't start that. Of course they did it.'" He said, 'Where's our proof? We can't prove either of them ever set foot inside the Clutter house!'" (3.260)
Dewey proves to be a patient and effective interrogator who eventually gets Perry to confess and spill the details of the murders. Through most of the book, we see him as a methodical, careful person—not unemotional, but focused on the task at hand. But we get a sense of his emotional depth as he watches Perry being led to the gallows:
The preceding execution had not disturbed him. He had never had much use for Hickock, who seemed to him "a small-time chiseler who got out of his depth, empty and worthless." But Smith, though he was the true murderer, aroused another response, for Perry possessed a quality, the aura of an exiled animal, a creature walking wounded, that the detective could not disregard. (4.313)
These men are the other main investigators in the Clutter case. Nye tracks down Hickock's parents, and along with Church, interrogates Dick. Duntz assists Dewey in his interrogation of Perry. Critics of Capote's account have said that Dewey's role in the capture of the killers was inflated in the book because of his friendship with Capote, and that other lawmen were actually more important to the investigation.