In this passage, Jesus was transfigured before 3 of His disciples. He leads them up a high mountain without the other disciples. There is no description if Jesus was telling them why they had to follow Him up the mountain. They might have wondered what this was going to be all about. When Jesus calls us, we have to be willing to go up our mountains to have a special encounter with Him.
As Jesus' appearance changes, Peter immediately resorts to talking about setting up tents for Moses, Elijah, and Jesus. Peter's response is to turn back to his familiar ways of religion, perhaps pointing to the Festival of Booths that was celebrated each year. However, the point of the transfiguration is God doing something new...the revelation of the work of the Christ and not a return to the old religious ways that were practiced in the Old Testament. Yet, even we are similar to Peter. We think to encounter God, we have to go back to our legalistic, religious ways of service. God wants to do something new in us today and it's not a rehash of what He did in previous generations.
As Peter was speaking, the voice of God calls out and the disciples are terrified. Jesus reaches down, touches them, and says, "Get up and don't be afraid!" In that moment, He refocuses their attention on Him alone. Likewise, when we are called by God, we might be afraid of the future, but Jesus still tells us to "Get up and don't be afraid!"