Anyway, the whole thing got me thinking. The same 'acting out of character' thing can be used when writing or editing. For writing, it can be one way to show that something really is wrong with your character. If she's normally pleasant and then flies off the handle, it would show the reader something is wrong, or at least foreshadow the possibility she isn't quite herself. And we've probably all seen instances where someone we know is bad acts all sweetness and light - usually right before they shove a knife in someone else's back (either figuratively or literally).
One example I can think of to illustrate the whole 'out of character thing' is the personality of Harry Potter in 'Order of the Phoenix'. Angst-ridden teenager didn't seem like him, and I've talked to a lot of people who found it irritating. I found it annoying myself until I read far enough to realize how JK was trying to show that Harry's life was such a mess it was making him behave like a toad. I don't know whether she consciously did that or not, but once I figured out what was happening, it worked perfectly. Once he felt like he was in control of his life again, his personality came back online.
On the other hand, keeping your characters in character is necessary for the flow of the story. You shouldn't really have them act one way for most of the book and then act totally different in other areas without a good reason (like the ones mentioned above). I know I'm guilty of this during my first draft stages. Early on, I'm still getting to know my characters. Hell, I didn't really hear Vic's voice until most of the way through Nano, and now I have to go back. By the time I'm done editing, he'll sound and act the same throughout the book (unless I need to make him otherwise for the story). Like changing a character's name mid-book. You can do it, but you have to make sure you go back and rename him all the way through. I just wish realigning personalities was as easy as Find/Replace.
Have you ever used this? Have you read anything where a writer used this device? Inquiring minds... and all that.