To participate with Jesus in the mission he calls his Apostles to, and to make that a template for our day, is to be in tune enough with the Spirit that when difficult times arrive we can expect the Spirit to give us the words to say.
Jesus predicted rough patches — which is mild language for overt opposition. Their opposition would come from the seats of power in Galilee and Judea. His followers would be asked to testify about what they were doing. In such settings, Jesus said, don’t worry about what to or (and this is important) how to say it: words will be given, and they will be given by the Spirit.

Doug Pagitt’s recent book on preaching, Preaching Re-imagined, while I don’t agree with the whole of his proposal and you can see my previous posts on that book, anchors its ideas in this sort of thing by Jesus: “speaching” (as he calls it) is too formal. It does prepare very carefully both on the “what” and the “how”. Perhaps we can learn from Jesus here to trust in the Spirit for the right words — both in content and form.
This isn’t a stupid suggestion that we give up preparation of sermons, but instead that we let our preparation be swallowed in the Spirit’s guidance.
Genuine Jesus missional work is prompted through and through by the Spirit.