There's also the issue of making it interesting. If it doesn't hold a person's attention then there's really no point in writing fiction. From my years of reading I would say that the key to this is really in the characters themselves. A writer can make errors in time, accidentally have contradictions or other problems that editing should have caught, but if the characters are real and engaging, then the work will still hold my attention. The character himself can be a bore, but as long as he's handled by a competent writer then he can still be interesting.
This is where my problem lies. I want my characters to be happy, but then they end up being flat. I find myself making my characters react to situations the way I would like to react, and that's not particularly believable, nor is it particularly interesting. It borders on preachy if I'm not careful.
A character should have a past, even if that past isn't discussed in the work. But, I'm too kind to mine. I don't want to subject them to a childhood spent fending for themselves because their parents were crack addicts who didn't have enough strength to put their children before their addictions. I don't want her to have been lured by a high school hottie into a small room just to have him cajole her into letting him touch her, and then force himself on her for his own pleasure, only to discard her once he'd relieved himself. Or for him to have sat at a lunch table in Jr. High, focussing on his cold sandwich while other students ridiculed him, laughing among themselves at who could say the most hurtful thing, standing behind him with their mouth close to his ear almost shouting, "Why are you here? Nobody wants you here, just go somewhere else. Can't you see that nobody wants you here?" All the while he can only look at his cold sandwich and eat it at fast as he could, his face burning with shame. Knowing from experience that looking around the cafeteria for an ally would be pointless, knowing that even the teachers would not intercede.
The things that make a person interesting and real are not just lovely experiences and coddled childhoods. Experience makes a person who they are, and experience isn't always pleasant. A well-adjusted, perfectly mannered man who respects everybody around him and handles adversity with a calm, pleasant demeanor before washing his hands, drying them neatly on a towel, which he hangs on the rack where it belongs, followed by a cup of tea in an immaculate house among delightful friends is not exactly riveting. People have faults and faults come from and cause unpleasant experiences. For a person who has spent the better part of his adult life being kind to others, creating characters who have suffered is not easy. But, if the goal is to write fiction, then the requirement is to have characters who have lived in the world. My biggest challenge so far.