Do you write in character much? Do you take measures to ensure that your characters are internally consistent - that is, that they do and say and like and buy and discard things in a way that you find believable? That is consistent with the person your reader imagines them to be?
This is not to say your characters can't surprise us. But they need to surprise us in a believable way, one that is consistent with what you know about them. Cinderella can't turn to Drizella at the end and say "Take that, you uppity bitch." There's not much that will make me put a book down and walk away, but having characters utter inconsistent dialogue is one of them. Much of what I read for pleasure is science fiction and fantasy, so I do a lot of suspending disbelief - but if I have to suspend it a half-dozen different ways, I get tired, and I move on.
Did you see The Muppet Wizard of Oz? I was looking forward to it. I admit it. I'm a Muppet fan. I had a Dr. Teeth poster on my wall until college. I can quote the original Muppet Movie and name most of the cameos. In order. I rank Muppet Christmas Carol in my top 25 favorite movies of all time. But this movie lost me ten minutes into Oz. (Disclaimer: I'm about to discuss the believability of characters made of felt. So noted.) First, the witches portrayed by embodiments of Miss Piggy just made no sense. Were they good? Bad? Each sentence spoken by the Glinda-like character could have been attributed to a different member of the scene. But the worst offense came later when we met the Tin Thing.
OK. Gonzo as Tin Thing was cute. The set up was believable - he's fused into a lot of gadgets. Then Toto (Pepe the Prawn, about whom don't get me started) touches one of Gonzo's handles and asks what it does, and Gonzo replies "Nothing. Those are my nipples."
Forgive me, but that line is a disbelief-burster. I suspend so that my film universe includes the Muppets as real characters, then one of them speaks a line completely inconsistent with who I've believed "him" to be. The nipple line is for the writer, not the audience. Trust is broken. Channel changed. Opinion is divided on this issue, but I think it's fair to say that those with more investment in and attachment to the characters probably had the hardest time with it. Please don't confuse this with nostalgia or resistance to change. This is about a character losing believability as a character. It could be Wonder Woman praying to be rescued, Chandler Bing starting an episode by writing out a career plan. You lose trust, and therefore interest, in the character before you.
NOTE: I haven't seen Muppet/Oz past FozzieLion's appearance. I have the whole thing on tape, and I have no desire to pop it in the VCR.
What does any of this have to do with my writing? Well, recall when I said I'd learned the difference between a manuscript and a series of poems? I'll put these two thoughts together next.