Angels often startle people and make them afraid when they appear

Daniel 10:4-8  On the twenty-fourth day of the first month, as I was standing on the bank of the great river, the Tigris, I looked up and there before me was a man dressed in linen, with a belt of fine gold from Uphaz around his waist. His body was like topaz, his face like lightning, his eyes like flaming torches, his arms and legs like the gleam of burnished bronze, and his voice like the sound of a multitude. I, Daniel, was the only one who saw the vision; those who were with me did not see it, but such terror overwhelmed them that they fled and hid themselves. So I was left alone, gazing at this great vision; I had no strength left, my face turned deathly pale and I was helpless.
Luke 1:12 When Zechariah saw him, he was startled and was gripped with fear. 

Luke 2:9 An angel of the Lord appeared to them (the shepherds), and the glory of the Lord shone around them, and they were terrified.

Matthew 29:2-4 There was a violent earthquake, for an angel of the Lord came down from heaven and, going to the tomb, rolled back the stone and sat on it. His appearance was like lightning, and his clothes were white as snow. The guards were so afraid of him that they shook and became like dead men.

Gill notes how in the case of Zechariah he was troubled, and fear fell upon him; for such appearances of angels were not now so common as formerly: and when they were more usual, generally had such effects on the minds, even of good men and in the case of the shepherds - at the sight of such a personage, and at such unusual light and glory about them: they ... not used to such appearances ... were awed with the majesty of God, of which these were symbols, and were conscious to themselves of their own sinfulness and frailty.

Angels are all in submission to Jesus Christ

1 Peter 3:22 ... who has gone into heaven and is at God's right hand - with angels, authorities and powers in submission to him. 

After his resurrection Jesus ascended to heaven where he now sits on God's right hand. Every angel and every authority, clean and unclean, is subject and submissive to him. He is their Lord.

Angels are to be honoured but not worshipped

Revelation 19:10 At this I fell at his feet to worship him. But he said to me, "Don't do that! I am a fellow servant with you and with your brothers and sisters who hold to the testimony of Jesus. Worship God! For it is the Spirit of prophecy who bears testimony to Jesus." 

Angels are wonderful creatures and it is tempting, if one sees them in all their glory, to worship them but they are not to be worshipped. Only God is to be worshipped.

Angels come with bad news from God

Genesis 19:12, 13 The two men said to Lot, "Do you have anyone else here--sons-in-law, sons or daughters, or anyone else in the city who belongs to you? Get them out of here, because we are going to destroy this place. The outcry to the LORD against its people is so great that he has sent us to destroy it."
The angels come not only bearing good news but bad news too as in the story of Sodom and Gomorrah.

Angels come with good news from God

Genesis 19:9, 10a ... "Where is your wife Sarah?" they asked him. "There, in the tent," he said. Then one of them said, "I will surely return to you about this time next year, and Sarah your wife will have a son."

Luke 1:11-17 Then an angel of the Lord appeared to him, standing at the right side of the altar of incense. When Zechariah saw him, he was startled and was gripped with fear. But the angel said to him: "Do not be afraid, Zechariah; your prayer has been heard. Your wife Elizabeth will bear you a son, and you are to call him John. He will be a joy and delight to you, and many will rejoice because of his birth, for he will be great in the sight of the Lord. He is never to take wine or other fermented drink, and he will be filled with the Holy Spirit even before he is born. He will bring back many of the people of Israel to the Lord their God. And he will go on before the Lord, in the spirit and power of Elijah, to turn the hearts of the parents to their children and the disobedient to the wisdom of the righteous - to make ready a people prepared for the Lord."

Luke 1:19 The angel said to him, "I am Gabriel. I stand in the presence of God, and I have been sent to speak to you and to tell you this good news.

Luke 1:26-33 In the sixth month of Elizabeth's pregnancy, God sent the angel Gabriel to Nazareth, a town in Galilee, to a virgin pledged to be married to a man named Joseph, a descendant of David. The virgin's name was Mary. The angel went to her and said, "Greetings, you who are highly favored! The Lord is with you." Mary was greatly troubled at his words and wondered what kind of greeting this might be. But the angel said to her, "Do not be afraid, Mary; you have found favor with God. You will conceive and give birth to a son, and you are to call him Jesus. He will be great and will be called the Son of the Most High. The Lord God will give him the throne of his father David, and he will reign over Jacob's descendants forever; his kingdom will never end."

Three obvious examples are the announcements of the births of Isaac, John adn Jesus. Perhasp that of Samson in Judges 13 could be added.

Angels (messengers) bring messages from God and must be honoured

Genesis 18:2 Abraham looked up and saw three men standing nearby. When he saw them, he hurried from the entrance of his tent to meet them and bowed low to the ground.

In one of the earliest references to angels in the Bible they are spoken of as men, as that is what they looked like. In Genesis 18:13, 14, 17ff and especially 19:1 it becomes clear that these are more than men, however. Whether we understand the three as God and two angels or simply three angels, here are messengers from heaven, bringing good news and bad to Abraham.

Angels always obey God

Psalm 103:20, 21 Praise the LORD, you his angels, you mighty ones who do his bidding, who obey his word. Praise the LORD, all his heavenly hosts, you his servants who do his will.

Instinctively and immediately angels always obey God's commands. This is the main basis for the part of the Lord's prayer wherewe are to ask that God's will be done on earth as in heaven.