Family Background
Pick one of the character’s family members. In this instance, I chose a sibling. What comes to mind when that sibling thinks of the character? I got this idea from a free writing prompt: What scared you? I’m not sure how I decided to write from the POV of the sibling of a secondary character, but what I wrote gave me so much insight into the secondary character that he became quite easy to write.
(Yes, these are tips to get to know ALL your characters, not just the primary ones. I’m not suggesting you do each exercise for every character, but if you are struggling with a character, try one!)