If so then how is there free-will? It seems like if there is free-will then everything that happens does so only because of the choices made by people. Unless of course, God personally tells someone to do something.
How does prophecy work? Does it require the prophet to completely turn his/her free-will over to God, at which point, God would speak through that person? Or can God just speak through that person any time He wants without the consent of the prophet?
This question is a major focus of the teachings of the Church and many heresies. Rather than going into depth into the theology of it, perhaps one may consider the following:
God made Man in his image, with intellect and will.
God made Man so Man may have God has his Last End. We naturally desire to return to God to complete all our desires.
God is beyond our understanding, so if we have complete understanding, then we are in error. However, we can understand that God allows us freedom within the design of God. At any given situation, one could make a case for God doing nothing or God being responsible for all. God is the First Cause, the source of everything, and he sustains everything.
Most things in our lives we do not control at all. In fact, we could be said to be in control of nothing at all, except the slimmest faculty of our will, and then, only weakly.
Given our fallen state, we are sick, and our intellect and will suffer, and our bodies are dying. We do not experience life perfectly. It is because of this we have these sorts of questions. In this state, we need God in a way that is perhaps something which is not comparable to how we need God in perfection (the way the angels need God).
In The Spiritual Combat, it is written:
DISTRUST OF SELF is so absolutely requisite in the spiritual combat, that without this virtue we cannot expect to defeat our weakest passions, much less gain a complete victory. This important truth should be deeply imbedded in our hearts; for, although in ourselves we are nothing, we are too apt to overestimate our own abilities and to conclude falsely that we are of some importance. This vice springs from the corruption of our nature. But the more natural a thing is, the more difficult it is to be discovered.
But God, to Whom nothing is secret, looks upon this with horror, because it is His Will that we should be convinced we possess only that virtue and grace which comes from Him alone, and that without Him we are incapable of one meritorious thought. This distrust of our own strength is a gift from Heaven, bestowed by God on those He loves. It is granted sometimes through His holy inspiration, sometimes through severe afflictions, or almost insurmountable temptations and other ways which are unknown to us. Yet He expects that we will do everything within our power to obtain it. And we certainly will obtain it if, with the grace of God, we seriously employ the following four means.
First. We must mediate upon our own weakness. Consider the fact that, being nothing in ourselves, we cannot, without Divine assistance, accomplish the smallest good or advance the smallest step towards Heaven.
Second. We must beg God, with great humility and fervor, this eminent virtue which must come from Him alone. Let us begin by acknowledging not only that we do not possess it, but that of ourselves we are utterly incapable of acquiring it. Then let us cast ourselves at the feet of our Lord and earnestly beg Him to grant our request. We must do this with firm confidence that we will be heard if we patiently await the effect of our prayer, and persevere in it as long as it pleases Divine Providence,
Third. We must gradually accustom ourselves to distrust our own strength, to dread the illusions of our own mind, the strong tendency of our nature to sin, and the overwhelming number of enemies that surround us. Their subtlety, experience, and strength surpass ours, for they can transform themselves into Angels of light, and lie in ambush for us as we advance towards Heaven.
Fourth. As often as we commit a fault, we must examine ourselves in order to discover our vulnerable points. God permits us to fall only that we may gain a deeper insight into ourselves, that we may learn to despise ourselves as wretched creatures and to desire honestly to be disregarded by others. Without this we cannot hope to obtain distrust of self which is rooted in humility and the knowledge of our own weakness.
Whoever seeks to approach the eternal truth and fountain of all light must know himself thoroughly. He must not imitate the pride of those who obtain no other knowledge than what their sins provide, and who begin to open their eyes only when they are plunged into some disgraceful and unforeseen debacle. This happens through God's permission that they may know their own weakness, and, by sad experience, learn not to rely on their own strength. God seldom supplies so severe a remedy against their presumption unless other means have failed.
Briefly, He permits persons to sin more or less grievously in proportion to their pride, and, if there were any as free from pride as the Blessed Virgin, I dare say, they would never fall. As often as you commit a fault, therefore, immediately strive to probe your inner consciousness; earnestly beg our Lord to enlighten you, that you may see yourself as you are in His sight, and presume no more on your strength, otherwise you will fall again into the same faults, or perhaps much greater ones to the eternal ruin of your soul.
It is clear that that regardless of the intricacies of the theology of free will and grace, that the best and only good thing we can do is to trust in God, and allow God's grace to guide us. Once we presume to rely on our own will and our own intellect, our weaknesses and mortality will be thrown in our face, even if we try to ignore it.
Prophets, in the sense that St. John was a prophet, answer God's call, and although they may not be too pleased with it, all the holy prophets seemed to have accepted it and it was not "involuntary" any more than Mary's acceptance of the Incarnation or Adam's sin.