The difference between this type of witnessing and the kind which I call "evangelizing" or "preaching" is that God opens the door for us to a particular person or circumstance. And as you say, He gives us the words if we will only listen to Him and surrender to His leading.
Evangelists and preachers are gifted by the Spirit to speak to audiences where some have ears to hear and others do not. The hearers have not been "primed" by God. They are sowing the fields widely (Matthew 13 - Parable of the Sower.)
And they are gifted by the Spirit for the task of sowing. By contrast, we may be gifted for the task of clearing fields, tilling, watering, aerating, weeding, etc.
And God hath set some in the church, first apostles, secondarily prophets, thirdly teachers, after that miracles, then gifts of healings, helps, governments, diversities of tongues.
[Are] all apostles? [are] all prophets? [are] all teachers? [are] all workers of miracles? Have all the gifts of healing? do all speak with tongues? do all interpret? - I Cor 12:27-30
But the one who is following the Spirit has God's blessed assurance:
For the which cause I also suffer these things: nevertheless I am not ashamed: for I know whom I have believed, and am persuaded that he is able to keep that which I have committed unto him against that day. - 2 Timothy 1:10-12
For the Holy Ghost shall teach you in the same hour what ye ought to say. - Luke 12:12
Then Peter, filled with the Holy Ghost, said unto them, Ye rulers of the people, and elders of Israel, - Acts 4:8
TRUE. TRUE.
AGREED.
THX THX.