Hmm, I wouldn't look at it like that. No personality is "inconsistent", really. Even if a person's behaviour changes a lot, that probably just makes them a moody person, which is also a consistent trait. There are probably cases of people who change their mind a lot, but that should be only temporary.
Of course, most fictional characters probably are simple and streamlined, since most fictional characters have characterizations that aren't complex beyond 5 times of text, and all characters are dependent on their writer's ability to convey them. But if a character is well-written, and the writer conveys them well - they still don't visibly have anything close to all the dimensions that a real person has, but that's natural since you only see these characters for a limited time, and usually from a limited perspective.
What I mean to say is that any fictional character, much like any person you don't know well yet, is "incomplete" in their characterization. With real people, you can try to get to know them better. With fictional characters, after you've seen all the stories they are in, you just have to assume that the writer did the best possible job portraying them and fill in the gaps. That's where Alternate Character Interpretation
comes from. If you only have limited knowledge of a character, but you treat them as a real person, you have to try to figure out what they must be like if they acted the way they did in a certain situation. Just like what you do with a real person, really. You have to take their word regarding what they say about themselves, the person might not understand him- or herself perfectly, either, and in the end, you automatically form your own interpretation of who this person is. My image of myself is different than the image my mother has of me, or anyone else who's ever dealt with me. And any fictional character I create, will face the same treatment.
Iiiiii guess the point I'm trying to make is that, if you decide to treat a fictional personality like a real personality, they are pretty much the same except that you usually can't interact with the fictional one to find out more about them, and that a real person probably can't be just a victim or result of bad writing or different authors.