Disagree with the whole premise here. Throwing out character sheets is how you end up with people who do things because the plot needs them to, not because they're real. The secret only works because the backstory is already in your head, even if it never hits the page. Take away the history and the lie has nothing to push against, so the reader just gets a vague mood instead of a person. That teacher got a better scene out of you because you already knew the history and finally stopped dumping it on the page, not because the history didn't matter. Keep the sheets. Just stop handing them to the reader.

Did you ever watch someone measure a board like five times and still get it wrong? My buddy tried building a shelf last month and kept coming up short because he thought a 2x4 was really 2 inches by 4 inches. He measured the space, bought the wood, and then the boards didn't fit the brackets he got. Turns out the actual size is like 1.5 by 3.5 after they dry and get planed down. He was so mad, he called the store asking if they sold him the wrong stuff. The guy just laughed and said welcome to lumber, man.