We all need those people in our lives. We need those people that can stand with us and encourage us and even be that in-between person when we have not been on par with God's voice or will. That person to come along side us and pull us up and share Godly wisdom or ever a word of knowledge or prophecy.
If you are a Nathan in someone's life, the only fear that you should have in sharing what God wants you to share with your King David is a holy fear. God has placed you in that position in your friend or spouses life for a reason. It can be uncomfortable at times but trust the Holy Spirit. Trust that as you are walking with Christ and your focus is on Him that He will guide your words and use you. Do it in love. Do it without agenda and be obedient.
2 Samuel 7:3-5
Nathan replied to the king, "Whatever you have in mind, go ahead and do it, for the Lord is with you." That night the word of the Lord came to Nathan, saying:
"Go and tell my servant David, 'This is what the Lord says: Are you the one to build me a house to dwell in?
"Why do you look at the speck of sawdust in your brother's eye and pay no attention to the plank in your own eye? How can you say to your brother, 'Let me take the speck out of your eye,' when all the time there is a plank in your own eye? You hypocrite, first take the plank out of your own eye, and then you will see clearly to remove the speck from your brother's eye.
1 Corinthians 16:14
Do everything in love.