After the death of the Prophet Joesph when the Saints were left on their own to follow "in faith" instead of following "by example" so to speak, there were many who had stepped up to "assume" the mantel.
When Brigham Young encountered a meeting of the body of the Saints that had gathered to hear Sydney Rigden preach that he was the prophets chosen spokesperson and therefore he should lead the Saints, Brother Brigham stood and addressed the body which included the quorum of the Twelve.
Instead of asking at first who the saints should appoint as their next leader, he said something really interesting. He told the crowd that no matter who they picked, the quorum who he believed now held the keys or Rigden, who claimed to be the prophets successor - they better follow whomever they pick as they did Joseph and not just follow until they are told something they don't agree with and change their minds.
Later he would say that it takes more then a belief of testimony of the gospel of Jesus Christ, the church or the prophet. Just having those makes us no better then Satan and his angels who not only have a testimony of those things but an actual belief based on knowledge which is something we don't have, we only have faith. If they didn't they wouldn't be working do hard to destroy it. Its takes actions. It takes putting your actions behind that faith and doing.
He also said that no matter what it is the Spirit that matter most. No matter what it is the Spirit that will confirm truth of all things.
If we choose to follow the Prophet only when we agree with him, what is the point of having a prophet? God does not require blind faith as some would say following the prophet implies. He does expect complete faith not half hearted faith when it sounds good. He expects us to try our best. That's the key. He expects us to pray, use the Spirit and try our best. Yes, we need to pray about things and yes we need to seek the guidance of the Holy Ghost in doing so, but if we have a testimony of the restored gospel then part of that is in the calling and sacred duties of the prophet.
When the Prophet speaks as a man he is a man, for instance I'm sure there are things Pres Monson likes that I don't. Just because he likes a certain flavor of ice cream or something like that doesn't make it the inspired choice. But when he speaks as the Prophet he speaks for the Lord.