Backstory

Carolyn Jewel wrote:

...we are not writing a story about the backstory of our novel. We’re writing about what happens BECAUSE of the backstory.

And that's how you know what backstory information to include and when. Work in the information about why the character is acting the way she is when she acts in a way that doesn't make sense or seems odd or off-putting to the reader if he doesn't have that information.