For I will give you a mouth and wisdom, which all your adversaries shall not be able to gainsay nor resist.
Note 1 at Lu 21:15: This promise of the Lord giving us wisdom in our speech that all of our adversaries will not be able to deny or resist is promised specifically to those of us in the midst of persecution. However, the same truth applies in any situation where we are giving a witness for the Lord. It is God within us who enables us and makes us effective witnesses.
It's not that premeditating what we would say is bad in itself. If we had the mind of Christ, we could certainly receive direction on what we are supposed to say in advance. However, when given the time, there is a tendency to rely on our own wisdom instead of God's. By not premeditating our responses, it makes us totally dependent upon the Lord for direction, and that is right where the Lord would have us.