Perhaps it’s the way I write but, when I first start on a project, I know as little about the characters as the reader does. It takes me the better part of a book to realise exactly who they are and, consequently, there are a few identity issues that need addressing in subsequent drafts. – The problem was it took me over three years to realise this.
If a character has a lopsided smile, first introduced in chapter 28, I make sure that it is added, as appropriate, in the preceding chapters too. Phrases and words, their reaction to stress, all these need to be consistent to ensure continuity. Yes, of course a character evolves throughout the book, but it’s important that this growth appears organic and not governed by the whims of the author.
One of the other big issues I used to have with my characters is how they react to situations. – Belief and immersion are suspended for me if, an hour after losing a loved one, the character is cracking jokes, their beloved forgotten. – The book I’m reading now is a thriller by a superb author, but a character has just announced to the protagonist that he is well over a hundred years old, but is spry, fit and young looking. – It’s set in our modern day world, and the protagonist, Liz, never reacted in the slightest. – The author may have got away with this mistake (and I feel it was an oversight) but I doubt you or I would, especially if we’re running the gauntlet of literary agents.
But you’ll miss more mistakes than you’ll find, and that’s why you need a bunch of betas who are pedants, not sycophants.