Jesus told his disciples that in difficult times that were coming, they should not prepare how to defend themselves. The Spirit would give them the words they would need. In the same way, sometimes we need to empty our own hearts of the words we have prepared to say and let God speak through us.
Based on Matthew 10:16-23:
Jesus said to his Apostles, ‘See, I am sending you out like sheep into the midst of wolves; so be wise as serpents and innocent as doves. Beware of them, for they will hand you over to councils and flog you in their synagogues; and you will be dragged before governors and kings because of me, as a testimony to them and the Gentiles. When they hand you over, do not worry about how you are to speak or what you are to say; for what you are to say will be given to you at that time; for it is not you who speak, but the Spirit of your Father speaking through you. Brother will betray brother to death, and a father his child, and children will rise against parents and have them put to death; and you will be hated by all because of my name. But the one who endures to the end will be saved. When they persecute you in one town, flee to the next; for truly I tell you, you will not have gone through all the towns of Israel before the Son of Man comes.
(New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org)