‘And he gave order to the centurion that he should be kept in charge, and should have indulgence, and not to forbid any of his friends to minister to him.'
So he gave orders to the centurion that Paul should be held in charge, but with a great deal of indulgence given to him so that there should be no limit on his friends ‘ministering to him'. It was normal for prisoners to be fed and provided for by their friends, so Luke clearly saw the courtesy extended to Paul as something extra, as giving him considerable leeway.
This would mean that under the protection of Rome Paul could see any brethren who wished to come to see him and could teach them to his heart's content. He was still in a position in complete safety to proclaim the word. At this time when there was so much trouble in Caesarea this would have been invaluable to the church there. People could have been popping in and out to see Paul all day and every day. It is therefore difficult to see why some see Paul as ‘inactive' at this time. He was probably as active as ever in the preaching of the word.
Meanwhile any further trial was in suspense. The Sanhedrin felt thwarted but knew their man and therefore that they would probably not get any further with him, and were not over concerned as long as Paul was not released. And Felix intended to do nothing at all. By keeping Paul in ‘friendly detention' he was preventing ferment and yet frustrating Ananias, which he probably enjoyed.