I’m a pretty lackadaisical RPer, I approach it like improv. I make a character I like and then I try to imagine what they are thinking as they go about their quests and gameplay, how they might feel being an orc or a gnome or a mage doing thus and so and make up little stories about them as I go, and eventually it turns into a sort of personality for them. then I talk to other people who are playing in character and have my character interact with them based on that personality.
Other people like to write a full story ahead of time and know everything about their character’s background down to what their favorite color is before engaging with others, and check it against the written lore to be sure it doesn’t break any canon. It depends on what you are most comfortable with and how far you want to go with it really.