This is also Luke"s first reference to an angel appearing. He evidently "materialized" beside the altar as Zechariah performed his duty of presenting incense on the altar and then prostrating himself in prayer. [1] Obviously God took the initiative at the time He chose to reveal what He was about to do. This was an angel from the Lord rather than the Angel of the Lord (cf. Luke 1:19). The right side of the altar may indicate the side of favor and honor, implying that the angel was bringing good news. Angelic appearances always indicated important events in the Old Testament (cf. Genesis 16:10-11; Judges 13:3-21). [source][source][source]
Zechariah"s reaction was violent because for the first time, presumably, he met a supernatural person face to face. This was the typical reaction of people in such situations (cf. Luke 1:29; Luke 5:8-10; et al.). [source][source][source]