From the book “20 Answers: Angels & Demons” by Father Mike Driscoll:
Although they have no material form in themselves, angels may, for the sake of human beings with whom they interact, take on a physical appearance by controlling the elements around them. They can do this by manipulating physical elements such as air, sunlight, and the dust of the earth. Such angelic appearances, though truly physical, do not constitute living bodies. Angels may also communicate through visions, in which there is no actual physical appearance, as in St. Joseph’s dream and the vision of St. John described in the book of Revelation. Angels also have the ability to give us ideas and inspirations, just as other human beings do; the difference is that angels do so in a nonverbal way.
Angels cannot see the future as God can, though they may sometimes appear to us to do so in two ways. First, God may reveal future events to them as He did to the prophets, which the angels may in turn proclaim to human beings (e.g., the destruction of Sodom and Gomorrah in Genesis 18-19, or the births of John the Baptist and of Jesus in Luke 1). Second, because they are capable of focusing their attention on any external events occurring in the world, angels can predict many events that human beings cannot. For example, someone’s guardian angel may know of a dangerous situation that the person cannot see, and may give him an inspiration to avoid the danger. The angel does not see the future, but does have a wider view of events that may affect the person.
“Remember that you have an Angel as a companion, guardian, and friend. If you wish to please Jesus and Mary, obey your Guardian Angel’s inspirations.” -Saint John Bosco
From the book “Joy to the World” by Dr. Scott Hahn:
In the Old Testament, angels appear often, as do other pure spirits, such as watchers (Daniel 4:14), cherubim (Genesis 3:24), and seraphim (Isaiah 6:2). They serve as guardians and guides, messengers and catalysts. Angels rescue Hagar (Genesis 16:7) and visit judgment upon Sodom (Genesis 19). They “go before” Israel, leading the chosen people into the Holy Land (Exodus 32:34). They bring God’s word to the prophets (1 Kings 13:18). Angels are mediators (Job 33:23), deliverers (Daniel 3:49-50), redeemers (Genesis 48:16), warriors (2 Chronicles 32:21), agents of creation (Psalm 104:4), and agents of destruction (2 Samuel 24:16).
No matter what the angels were doing, however, they were simultaneously worshipping. When Jesus spoke about guardian angels, He emphasized that even as they were guarding “little ones”, they were also worshipping the Father in heaven (see Matthew 18:10). Worship is the primary activity of all pure spirits. Worship is what angels do.
“Holy angel, you adore the Eternal Father’s face, as you always see it. Because His supreme goodness committed you to caring for my soul, rescue it always by His grace. Enlighten my soul in the darkness, console my soul in its pain, warm my soul in the cold, defend my soul in its temptations, and rule over my soul for the rest of my life. Deign to pray with me. Because my prayers are cold and languishing, set them on fire with your burning love, and carry them up to the throne of God to offer them to Him.” –Saint Charles Borromeo
“Because you have made the LORD your refuge, the Most High your habitation, no evil shall befall you, no scourge come near your tent. For He will give His angels charge of you to guard you in all your ways. On their hands they will bear you up, lest you dash your foot against a stone.” (Psalm 91:9-12)
Above all, you should honor your guardian angel for his constant assistance. And you should never do anything in the sight of this angel which you would be ashamed to do before another human being. Alas for you if your guardian angel should find you unworthy of the care which he bestows upon you!
There are, however, many virtues which bring delight to these holy angels. They deeply desire to see these virtues flourish in our lives and in our hearts. Such things which please the angels include sobriety, chastity, voluntary poverty, and heartfelt prayers poured forth with tears and sighs. Above all these other virtues, though, they are delighted with peacefulness and fraternal charity. And our holy angels help us to attain all of these virtues through their unceasing intercession and guidance.
O my soul, you are the most beautiful and lovely image of your Creator! How greatly are you loved by God, and how deeply esteemed you are by His angels! If you are ever mindful of not offending these glorious beings or causing them sorrow and distress, you shall then easily avoid the foul and vile stain of sin. For if there is such immense joy amongst the angels in heaven over the conversion of one sinner, how much sorrow must your own guardian angel feel whenever he sees you slip into sin and thus be separated from divine grace?
My soul, seek therefore to adorn yourself with all those virtues which exhilarate the angels and glorify the Creator! For, emulating the virtues of the angels, you shall surely come to enjoy the sweetest fruit of eternal bliss in their glorious company.