Basics of Collision help. Working with character controllers.

I have a bit of C++ experience, but this is my first attempt at anything Unity. Currently the plan was to make a small, 3d, adventure game, and i'm working on being able to interact with NPC's. I have just two rectangles in the scene, Player character and NPC. PC is using the third person player controller, and I have a small invisible rectangle parented to him, being used as a trigger. My intention is to check for an NPC which is colliding within this trigger, and if there is, allow them to interact. The NPC is just another square, using a character controller as well.