In a long conversation with another writer recently, we discussed why some reading experiences are memorable and others aren't.
I think we remember what we connect with emotionally. Even an exciting new idea, which we would ordinarily classify as cognitive, not emotional. Many, many times I think I remember exactly where I came across a new idea - I can "see" it somewhere in the upper third of a left page, and remember it word for word. But I'm not an underliner, and when I go back to look for it, it's not there. There's only a word or a phrase of the sentence I was sure I had remembered exactly. No, that word or phrase had made connections in my brain, and I had myself created the entire sentence I thought I had read.
I think we also respond to the emotion of the writer. If writers are truly engaged in the character, the words they use, the length of sentences, will engage the reader and make even a minor character memorable. Last night I read a chapter that mentioned a name which, because of the circumstances, should have appeared in a previous chapter. I skimmed that earlier chapter three times, but couldn't find her. I don't know who she is this morning. I also read a chapter from a minor (at least, so far) character's point of view in Alex Grecian's The Devil's Workshop. I remember this morning that he dropped and broke a tea cup near the beginning of the chapter and that he felt useful, at last, when he cleaned up the mess he'd made at the end of the chapter. I remember how he walked up the stairs, how helpless he felt when he was asked not to open a door. And a particular moment when he leaned his head against the door, hoping that would somehow help him see what was on the other side. I believe that Grecian had to have been inside that character during every moment of that chapter, feeling the emotions he was feeling on each step of the stairs.
If I reread that chapter now, I may find that what I remember is, again, not exactly what's on the page. But what is on the page allowed me to be inside that character, to re-create and believe in him.