Talk to them about an interesting subject with them, let them talk and tell you the story, then ask them questions, silly questions may work better to get different expression! then focus on their reactions, face expressions then click!
All you need to do is to have your camera fixed on your hand, a tripod/table, maybe have a remote or just simply put your finger on it ready to click while they’re talking and telling you their never ending stories, be ready and wait for the expression may be while they’re looking at you or not!
Distract them from the camera, Capture that natural face expression!
Location: Although it is always ideal to set these conversations by the window to have the best light, sometimes you may just need to follow them where they like to go and carry on talking to them!