This might be a bit controversial, but I tell my students that, in my personal experience, (real-life) people don't change. That the CORE of who they are (personality, belief system, the way they view the world) is often established at a pretty early age (or, they're even born with it, in the case of personality). And that they rarely, if ever, veer from that main core.
Sure, people grow and learn and stretch themselves. Absolutely. And sure, people can have religious experiences, which often are accompanied by significant alterations in their beliefs. But most people I know react the same way to the same situations time and time again (myself, included). In fact, I could present a hypothetical situation and just about predict how a certain person might react.
Maya Angelou has a fantastic quote: "The first time someone tells you who they are, believe them." To me, that means that people show you who they are, and they will CONTINUE to act that way, time and time again, so we'd better brace ourselves for it.
Another great quote comes from Don Draper, the fictional character in Mad Men. I jotted this down from last week's episode: "People tell you who they are, but we ignore it. Because we want them to be who 'we' want them to be." Again, meaning that as much as we want someone to change, they most likely won't. They'll most likely continue to be exactly who they always were.
So, back to fiction -- just as people tend not to change who they truly are, at their core, so fictional characters shouldn't either. At least, not without justification. Here's an exaggerated example: if Sally starts out in your novel with a passive, meek, introverted personality, she probably won't end up at the end of the novel with an assertive, sanguine, outgoing personality -- unless something significant has occurred within those pages to justify the "change."
The reader gets to know a character better with each page he/she turns. With every sentence, the writer is building a background for the character, laying a specific foundation. So if, suddenly, for no reason, the character changes drastically - does something completely OUT of character - the reader will know that. And will be frustrated by it. Because, suddenly, the characters don't seem "real." They seem "written."
So, what do you think? Are real-life people - or characters - capable of drastic changes in their lives that are lasting, sustainable? I'd love to hear your thoughts!