To OOC or not to OOC

Sometimes people say, that a character is out of character. But what is “in character”, in this case?

I have found a good quote about this phenomena, from the Depending on the Writer page on the TV Tropes.

The characters and their various interactions with other characters vary from writer to writer. Pinkie Pie can either be an insane stalker-like lunatic or a hidden genius, Fluttershy can ranged from being mildly socially awkward to a nervous wreck, and Spike's crush on Rarity can be almost complete devotion to her, simply non-existent or anything in between. Even Scootaloo's idolization of Rainbow Dash can range from Fangirl to not treating her any differently from other adults.

Illustration of out of character characters, source. Or, maybe, not so out of character...