I'm looking for any manga that has a story arc where a character acts in a way that is inconsistent with their normal personality. Especially ones where a tomboy acts girly or a fragile girl acts strong. Any other example such as when a lazy person acts diligent is also appreciated.

This is usually played for humorous effect, by shocked friends/associates. Comedy is what I am hoping for but other examples are welcome. I am not interested in slow character growth, the main point is that people should think that the character is acting like a different person. Going back to a hometown and surprising family is acceptable. Beauty transformations are not really what I want, but if the personality changes suddenly as well they are appreciated.

Some examples would be
Yamatonadeshiko Shichihenge
This happens to Sunako multiple times during the series.

Gokuraku Seishun Hockey Club
Chapter 23

Plot devices other than possession are good, and it can be the plot of the whole series, (common for body swap category) however, the perception that the person is acting out of character must be used for comedic/dramatic effect, for at least more than just one scene.

Fairy Cube - the main character gets to the fairy world, which can be creepy at times, and his body is possesed. Thus his behavior changes. It's a Yuki Kaori work so don't expect any comedy;
Love Luck - another body swarp, but this time it's a comedy. The interesting part is that not only the bodies change but also tha initial personalities;
Tokyo Crazy Paradise - this series is very long so the part you are looking for is somewhere in volume 5 I think. But it's really funny when the lead who is a tomboy develops (for a short while) another 2 personlities: one very girly, cute and innocent and another one more mature so to say ;

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I love vampires.

Orange Chocolate has body-swaps between the two main characters and when they're switched people, get totally weirded out (and despite the summary, the main point of the story so-far seems to be the body-swaps)

I can't think of any others at the moment, sorry!

Thank you those do seem like what I want.

I am already reading Orange Chocolate and I do like it very much, but it's not exactly what I mean because they go out of their way to try to be inconspicuous. I am looking more for situations in which the person acts suspiciously different, but they themselves don't really notice that their personality has changed. Maybe that is a little to selective though.