The contributors to the blog OnFiction profess to be doing "research on the psychology of fiction." If we take this to encompass broadly the increasing popularity of "cognitive theory" and neuroscience in the analysis of literature and our response to literature, "psychology of fiction" attempts to describe our reaction to fictional characters as if those characters were real people with minds, who, as a recent post at OnFiction has it, provoke us to "wonder what they are up to." In this view of the reading experience, "we readers imagine ourselves into the minds of characters as we run the simulation which is the literary story." So do writers, which accounts for the reported instances of characters "exhibiting apparently autonomous agency" during the composing process.
As a reader, I have never "imagined myself into the minds of characters" while running "the simulation which is the literary story." Neither can I really believe that anyone else has done this. In the first place, in most fiction that to any significant degree asks us to consider the mental life of its characters, we are not encouraged to imagine ourselves "into" their minds. Their mental life is presented to us explicitly, often in the narrative mode called the "free indirect" method, sometimes directly through a stream of consciousness, or near stream of consciousness, point of view. We don't have to "wonder what they are up to" because the author/narrator makes it perfectly plain what they are up to. Perhaps it is the case that in some first-person narratives we are invited to read between, or behind, the lines the narrator literally offers us, resulting in a perception of the narrator's state of mind to which even he/she has little access, but I don't think this relatively special circumstance is what the "psychology of fiction" generally emphasizes.
Second, in what way does "the simulation which is the literary story" differ or depart from the literary story itself? Are we being told that the "literary story" exists as a way for us to imagine the characters in other situations, situations the story doesn't relate? That the story is merely an excuse for us to wonder abstractly about the characters in all of their "autonomy"? Or is it that the "simulation" we "run" is just the story itself and that when we "imagine ourselves into the minds of characters" we are simply envisaging what it would be like to be these characters involved in this story? If "character" is so overridingly important in a work of fiction that we are led to detach it from all other elements of the work and regard the characters as real people we might ask over for drinks, then why bother with the other elements of the story? The author could just send us a character sketch of his protagonist, whom we could then imagine in any circumstance we'd like. And while I do believe some readers project themselves into the situations in which fictional characters are portrayed, this has more to do with the operations of the readers' minds than those allegedly at work in the characters' minds. Such readers are more engaged with the story in which the characters appear than with the characters themselves, certainly more than they're connected to the "minds" of those characters.
Similarly, I'm sure that some writers do experience their characters exhibiting "autonomy," taking the narrative in directions the author didn't anticipate, but I doubt that very often this is a result of the author dwelling in the characters' "minds." In my own on-and-off career as a fiction writer, I have had characters wander off the plotted path, but never because I could read in their minds that they thought it best to do so. Either the character's voice seemed to provoke a change in plan, or the story itself prompted a change in character, or the character just didn't seem in general to be the sort of character who would do that rather than this. Realizing that a narrative needs to be readjusted because one's preconceptions about a character's role in it have been altered is an aesthetically sound decision, but it has very little to do with remaining alert to the "psychology of fiction."
If anything, a fixation on "character," even more reductively on the "inner life" of a character, only makes our response to the whole work of fiction more impoverished. The very best fiction, like the very best art in general, widens our perspective on the possibilities of the form as a form. It helps us enhance our experience of fiction by remaining alert to all of the artistic choices the writer has made. In John Dewey's words, "The artist selected, simplified, clarified, abridged and condensed according to his interest. The beholder must go through these operations according to his point of view and interest." Isolating character as the most essential element in works of fiction potentially circumscribes our response to them, blinkers our awareness of the other aesthetic "operations" at work in the text. It inherently declares fiction to be this sort of thing--a way of meeting up with imaginary characters--rather than all the other things it might be. It renders stories and novels into case studies for psychologists rather than complex works of art.