But even the main actors can ad lib (and get in trouble for it.)
And the LORD said unto him, Who hath made man's mouth? or who maketh the dumb, or deaf, or the seeing, or the blind? have not I the LORD? Now therefore go, and I will be with thy mouth, and teach thee what thou shalt say.
And he said, O my Lord, send, I pray thee, by the hand [of him whom] thou wilt send.
And the anger of the LORD was kindled against Moses, and he said, [Is] not Aaron the Levite thy brother? I know that he can speak well. And also, behold, he cometh forth to meet thee: and when he seeth thee, he will be glad in his heart.
And thou shalt speak unto him, and put words in his mouth: and I will be with thy mouth, and with his mouth, and will teach you what ye shall do.
And he shall be thy spokesman unto the people: and he shall be, [even] he shall be to thee instead of a mouth, and thou shalt be to him instead of God.
And thou shalt take this rod in thine hand, wherewith thou shalt do signs. - Exodus 4:10-17
To God be the glory, not man, never man.
If by "ad lib" we mean a person does something that is not ordained by God from before the foundation of the world then that doesn't seem to make a lot of sense. At the moment of creation, God ordained His plan for life and time and space and all existence to occur. All of it. And so any "ad libbing" would likewise have been factored into God's prescience since, by definition, He is omniscient.
And if something were to occur that did not comport with God's plan of creation, He would change it (at that same moment of creation) to comply with His determinant will.
And we can't get around this "ad libbing" by saying "at the moment of creation" isn't a real moment because God most certainly did begin creation at some point in time, which he also created. Life is linear. Life moves according to God's decree. It began when God created life and it will end when God wraps up time.
And I don't think we can say both free will and predestination are true because they are polar opposites. Either men act independently of God or men fulfill the lives God has given them. As the WCF says in Chapter 3, "Of God's Eternal Decree" -- "God from all eternity, did, by the most wise and holy counsel of His own will, freely, and unchangeably ordain whatsoever comes to pass;[1] yet so, as thereby neither is God the author of sin,[2] nor is violence offered to the will of the creatures; nor is the liberty or contingency of second causes taken away, but rather established.[3]
So ultimately, it doesn't seem like there is such a thing as an "ad lib" because that would imply there is something outside the perfect control of the perfect Creator. Something would be autonomous; independent of God's sovereignty. But we know that God animates everything. Without His sustaining hand, the world would not exist.
If you guys have some time, a worthwhile read is the fairly succinct "CALVIN'S TREATISE ON PREDESTINATION"...
"By Providence, we mean, not an unconcerned sitting of God in heaven, from which He merely observes the things that are done in the world; but that all-active and all-concerned seatedness on His throne above, by which He governs the world which He Himself hath made. So that God, as viewed in the glass of His Providence, is not only the Maker of all things in a moment, but the perpetual Ruler of all things which He hath created. That Providence, therefore, which we ascribe to God, pertains as much to His operating hands as to His observing eyes. When, therefore, God is said to rule the world by His Providence, we do not merely mean that He maintains and preserves that order of nature which He had originally purposed in Himself, but that He holds and continues a peculiar care of every single creature that He has created. And true and certain is the fact, that as it was the wonderful wisdom of God that originally made the world, and disposed it in its present beautiful order, so, unless the omnipotent power of God, ever present, sustained it thus created and disposed it, it could not continue in its designed order and form one hour..."
As xzins said, this understanding is simply "trust and rest."
Thou makest darkness, and it is night: wherein all the beasts of the forest do creep forth. The young lions roar after their prey, and seek their meat from God. The sun ariseth, they gather themselves together, and lay them down in their dens. Man goeth forth unto his work and to his labour until the evening. O LORD, how manifold are thy works! in wisdom hast thou made them all: the earth is full of thy riches. So is this great and wide sea, wherein are things creeping innumerable, both small and great beasts. There go the ships: there is that leviathan, whom thou hast made to play therein. These wait all upon thee; that thou mayest give them their meat in due season. That thou givest them they gather: thou openest thine hand, they are filled with good. Thou hidest thy face, they are troubled: thou takest away their breath, they die, and return to their dust. Thou sendest forth thy spirit, they are created: and thou renewest the face of the earth. The glory of the LORD shall endure for ever: the LORD shall rejoice in his works. He looketh on the earth, and it trembleth: he toucheth the hills, and they smoke. I will sing unto the LORD as long as I live: I will sing praise to my God while I have my being." -- Psalm 104:19-33"He appointed the moon for seasons: the sun knoweth his going down.