"The first-person treatment is seriously self-limiting. The "I" must say all that is in his mind and heart. There is no good way to dimensionalize that "I." All psychic reactions are flattened out. With good cause, it is rarely attempted in the serious novel. Saul Bellow attempted it in 1953 [The Adventures of Augie March]. Because he is basically a craftsman, he sustains his lead character throughout the action, but then he, too, succumbs to a curious fate, which seems typical of most of the "I" characters in fiction, particularly mystery fiction.
"The character can be sustained for the length of the book, but soon the outline of the man begins to blur. Philip Marlowe, Mike Hammer -- regardless of merit in the handling, cleverness in treatment, they all blend into one faceless, characterless, depersonalized "I" remembered vaguely for only those common qualities of durability and determination. Think back to those better books written in the first person. and see how much more clearly you remember the secondary leads, those people who had the character-building advantage of third-person handling.
"Yet we continue to do first-person mysteries because it is a device which simplifies the problem of reader participation in the action. We accept the self-limitation and do everything we can to sustain the lead through the book, perfectly aware that the lead will sink back into anonymity as soon as the book is closed."
-- John D MacDonald, 1956, from "How a Character Becomes Believable," Chapter 15 of The Mystery Writer's Handbook, edited by Herbert Brean.
Reading this made me wonder if MacDonald ever read any Jim Thompson. There's definitely no "blur" in Thompson!