Make it easy on yourself–keep your characters in action and in relationship. Physical interaction and dialogue between two (or three) characters is one of the easiest ways to reveal deep traits. When we witness your hero counseling a 12-year-old runaway we will judge your hero’s level of compassion and concern, we will probably get a glimpse of her past and the wound that leaves her vulnerable, and depending upon the 12-year-old’s responses, we’ll know if she is street savvy or naive–in our eyes and in the eyes of the ‘tween.
When you begin writing the scene, you will likely identify with your hero and you will become very conscious of what she wants in each beat of interaction. Let’s assume your hero is the viewpoint character–meaning we will have access to her thoughts but not to the runaway’s thoughts.
But! Make sure you also view (and contemplate) the scene through the eyes of the runaway, too. You will continue with your hero as viewpoint character but you will also greatly increase your chances of creating a complex, surprising, driving scene. Fiction writers forget the concept of “parity” at their peril. Parity means equality–and in the case of your scene, it means you can draw energy from both your characters and inject plenty of zing into the scene.
Be aware: This concept of parity or equality is for you, the author, to apply. Do not mistake parity for the concept of “status”–that is about who has the standing, the drive, the power in the scene. For surprise effects, you can have moments of “reversed status”, when the status quo is turned upside down.