#novice ficcers (of which i've been one!) are often so enticed by that delicious vulnerability that they don't put in the work to justify it

[i-dreamed-i-had-a-son]

Exactly! A lot of writing relies on what is unsaid. A lot of fanfic is inspired by that and wants to put it into words; poorly written fanfic bites the apple and actually does it.

I see this with a lot of fic in which the author clearly has strong views on the characters and their motivations, and writes them into the story directly through character exposition, not realizing that the characters themselves would not be able to express them because they are simply not self-aware enough.

For an example I've noticed recently, take Jean Valjean. He is a complicated case--he is very introspective at points, but often only to evaluate his sin and criticize himself. When he feels emotions like fear, jealousy, or even love, the narration indicates he wouldn't be able to explain what was going on in his head if someone asked him.

Furthermore, he is very consistently portrayed as dissociating almost immediately when placed in a difficult situation, and goes into either a survival mode (i.e. his conversation with a suspicious Javert, his flight to the convent, his escape with Marius into the sewers), or, more frequently, a self-harming mode (his desperate escape attempts, stealing from the bishop and Petit Gervais, the brand incident, his implied-suicidal presence at the barricades, giving Javert his address...and many more). This is repeatedly described as animalistic and instinctual; it is not a conscious process that he could explain. JVJ has a lot of unprocessed trauma, and it comes out in these latter incidents, but rarely (save isolated, odd instances such as the Montparnasse traumadumping, and even that is in a distanced third-person manner) is he able to articulate anything of the feelings, reasoning, or experiences behind his actions.

So when I see a fic in which Jean Valjean is able to easily explain that he has "always felt so afraid," or to freely articulate his concern and care for someone else, it reads as inauthentic. Yes, perhaps we, the readers, know that his fear or affection are primary motivators in a given instance, but Valjean himself would not (or at the very least, would not admit it). It feels almost gratuitous to see him laid bare--his inner character is what makes him so compelling, precisely because it is deeply buried. The same is true of almost any character, and that tantalizing inner experience that tempts less-experienced writers to turn them inside-out; but sadly, in doing so, they inadvertently make the character unrecognizable.

there are some things a character should not be able to tell us about themselves EVEN with a gun to their head. depending on the character that could even expand to include "most" things