Charles Spurgeon Commentary Luke 1:8-12

"Now it came to pass, while he executed the priest`s office before God in the order of his course, according to the custom of the priest`s office, his lot was to enter into the temple of the Lord and burn incense. And the whole multitude of the people were praying without at the hour of incense. And there appeared unto him an angel of the Lord standing on the right side of altar of incense. And Zacharias was troubled when he saw [him], and fear fell upon him." — Luke 1:8-12 (ASV)

And it came to pass, that while he executed the priest's office before God in the order of his course, according to the custom of the priest's office, his lot was to burn incense when he went into the temple of the Lord. And the whole multitude of the people were praying without at the time of incense. And there appeared unto him an angel of the Lord standing on the right side of the altar of incense. And when Zacharias saw him, he was troubled, and fear fell upon him. (Luke 1:8–12)

Zacharias must have been astonished as he saw that strange visitant; no wonder that fear fell upon him.