This is one of my favorite writing exercises because it is so spontaneous. If you are having trouble with a character, take them off the page. What I mean is, introduce them to external stimuli and scenarios.
For example, how would your character react if you dropped him/her in the mall? What kind of shops would they visit first, the tattoo parlor or the Build-a-Bear workshop? What clothing stores would they prefer? Or maybe your character is not a mall rat and would find him/herself perfectly miserable. Good. Bombarding your character with external stimuli helps to develop and reveal their tastes and personalities.
Besides, it's fun!