It seems to me that most people stay pretty much the same. However, anecdote:
There was this time when I had just started in a new job, and there was this one coworker. She was just about the nastiest, meanest, grumpiest, most unpleasant and most unhelpful person I’ve ever encountered.
And yet, everyone else at this workplace seemed to have infinite patience with her. When she bitched, they never bitched back. They cut her endless slack. They always responded to her with smiles, no matter what. I didn’t get it. I just figured, well, people here must all be unusually sweet, except for her. I adjusted my own behavior towards this person accordingly, adopting the same patience, never engaging her in any kind of conflict. I wanted to fit in, and I didn’t want to cause any fuss. It seemed strange, though, especially since these other coworkers didn’t seem particularly saintly in other respects. They were otherwise normal. It seemed like this particular nasty person was getting special treatment from them. Of course, it did make the working environment much nicer than it would have been if the hostility had been flying both ways instead of just one. Still, I was puzzled. I didn’t bring it up with anyone, though.
After a few months of this, or thereabouts, one of the weirdest things I’ve ever experienced happened: Her personality suddenly flipped. Just like that, from one day to the next. She went from being nasty to sweet as a puppy. From grumpy, to cheerful and happy. From sniping at people, to helping them, caring about them and asking how their day was. Basically, from demon to the sweetest grandmother you’ve ever had.
And that’s when I understood why everyone had been so patient with her earlier: They must have seen this flip happen before, probably multiple times, and they knew that she would eventually turn back again from nasty to nice.
It was totally a Jekyll & Hyde kind of thing, and I’ve never seen anything quite like it. She stayed her nice self for the rest of the time I was at that job.