Sometimes we learn more from day-to-day life than we might think.
One of the most poignant conversations I had during my mission was only a few sentences. We had been invited at someone's home for dinner. This family was great. They all were wonderful to be around. However, they were also somewhat different from most other families that we visited. I believe the father was a NASCAR pit-stop mechanic of some kind, I never talked with him. He seemed very quiet, probably because  we seemed reluctant to talk with him as well. Anyway, as we were getting ready to have dinner one night we were setting the table, and I guess the mother drank sweet tea with her meals, so she brought out a bottle. I knew that this was against the teachings of the prophet of our church, but I wasn't going to say anything at that time. If you read in the New Testament, the Savior doesn't go around correcting people. In fact, sometimes he takes the opposite approach from that which the religious people of that time thought he should. Anyway, I was acting completely normal, and then I guess this sister looked at me in a way that told me that she saw me as one who would judge her and correct her for her bad habit, so she said "Elder, please just let me have this one thing. It is my one thing." She said something along those lines. And I was surprised, because in the first place I wasn't going to say anything, and secondly the manner in which she acknowledged it showed that there was a large amount of guilt she felt by doing this "one thing". Did she see me as some sort of moral authority over her home? Anyway, it didn't make any amount of sense to me why she did not give it up if she felt so incredibly guilty doing it, to the point that she had to directly ask me to let it alone without me so much as looking at her funny. Sin in the lives of everyday people can be a very interesting thing to observe.

Take what lesson you will from this.

I found through that experience that following the Spirit is the best way to fulfill what God sends you to people to do. The Spirit is the measure for us of "how close am I to doing what God wants?". We can't make one ourselves.

The key is not being afraid to speak up, because that will sometimes be required and also because fear interferes with our spiritual "antennae" and prevents us from knowing what it is that we do need to do. Whether we are to call someone out verbally, or just by our silent spiritual presence prompt some healthy introspection, is not our decision but the Father's. He will let us know through the Holy Spirit what we are to do. But we should keep our minds open to the idea that sometimes just our being there is enough to make people think. Sometimes we don't even have to know the person well, but they recognize something different about us. They see that we hold ourselves to a high standard. I believe that spiritual power is a tangible result of righteous living and you don't always need to consciously choose to use it in a specific way, but you do have to work hard to develop it and to keep it.