At the end of the day, the characters are the most critical part of your story. They’re the ones with which your readers empathise; they’re the ones your readers will finish the book to follow. Some people have told me to write backstories and investigate every detail of their lifestyle so I can use them in a way that is consistent.
Here’s my simple principle: whatever makes it into the window frame of your story has to either impact your characters somehow or at least impact a role in a way that will grab your reader’s attention.
Build up your new world. Play and have fun. Go crazy with them. Then when it comes to your story, just make sure that the parts are seen through the window frame matter to the characters. It’s not who the characters are at the beginning, it’s what they do within the timeframe of your story. Their actions are what people see of them. A personality whom is merely upset and looks downwards is very different from a character that stands tall and slaps the face of the other actor that said something nasty.