My experience over the years with working with people who are not actors or not trained actors is that you have to get to know them well enough to see what they have that's translatable onto the screen. So you're constantly calibrating to play to their strengths. And the key is to never ask them to do things that are beyond their abilities or are really far away from who they are at their core.
I remember opening my dad's closet and there were, like, 40 suits, every color of the rainbow, plaid and winter and summer. He had two jewelry boxes full of watches and lighters and cuff links. And just. . . he was that guy. He was probably unfulfilled in his life in many ways.