In every exchange, whether passing by someone and making eye contact, or having a good common discussion with someone, we effect each other. A person who is an agent of light, love and Spirit, will make a good change, no matter how small or big, in their exchanges with another. Light will always extinguish dark.
This is why we must act as righteous individuals, and not be acted upon. However, for an unrepentant, darkened individual, who chooses to be dark in spite of a righteous exchange with someone, they will in essence harden their hearts more, and become even worse than they were before.
Every action has an equal and opposite action or reaction, meaning that we can react negatively to a negative action, or we can act positively to a negative action. The same would hold true for the opposite.
We can act positively, and have a positive outcome on someone who is negative, even if we can’t see it, or notice the change from our exchange.
We must seek the Spirit in all we do, and listen to and obey His promptings early. In this knowledge we can effectively and joyfully be successful member missionaries, and effective at all we do in life.