Exodus 4:3, 7:8-13

When God made the staff of Moses turn into a snake, Moses ran away from the snake.  He quickly overcame his fear though and grabbed it by the tail per God’s instruction.  By the time Aaron and he got to show God’s power to Pharaoh, Moses probably had no/little fear of the snake.  Later, Aaron heard God say that the staff would turn into a snake when Pharoah asked for a miracle, so Aaron was forewarned.