Strike when you see this happen, and trust me, you will see it.
Use your skills not only as a photographer, but as a friend, a husband, wife, or fellow human to strike up a conversation, embrace a topic and pre-empt a reaction. Whether this is a laugh, a moment of thought or curiosity, there will always be that moment where your subject engages with your question, reacts to your joke and cognitively links your words to their reply. Strike when you see this happen, and trust me, you will see it.