Thanks for playing! Do you mean how to help them move on from the conversation? Basically you have to ask the "correct" or fitting questions depending on their personality :D
If you mean in the other screen - the hourglass of each human walking around starts blinking blue-green and emitting particles when there's the correct time to cut their souls. To slash, hit space or click with the left mouse button. If you find a "special" human (= the ones in color), you'll get forwarded to a conversation with them.
When the hourglass blinks red, it's too late and the soul is lost. (will add this to the description, thanks for pointing out it's not clear)