We expect the character to behave in a certain way, once we’ve been introduced and spent time with him. If he’s the serious type, then we expect serious behavior, not a stand-up comedian.
If she’s happy-go-lucky, then if she delivers a serious speech, we might be thrown off track.
On the other hand, if something or someone causes the character to change behavior in an understandable way, then we adapt to the temporary change.
Your task is to write a story in which, after you’ve introduced your protagonist, he changes behavior in a drastic, unexpected way.
Have fun with this one.