We could read into what is written here and suggest that when John says he was in the isle that is called Patmos "for the word of God and for the testimony of Jesus Christ," that he is telling us these were the charges that got him banished - that he was preaching the word and testifying of Christ.
According to "tradition," John was exiled to Patmos when he was in his extreme old age. However, while he was there he was told (Revelation 10:11) that he "must travel and prophesy to many peoples, and nations, and tongues, and kings." We have to wonder about this being fulfilled if John was so old at the time of the late-date composition. The key to this is in the Greek term translated "for" here. In other words, John was in Patmos IN ORDER to preach the Word and testifying of Christ. The word better supports no banishment rather than banishment.
John could have received the Revelation (or some of it) in portions throughout his ministry, giving him enough time to share the contents with the actual servants of the Lord (the other apostles?) since we have read in the first chapter that "The Revelation of Jesus Christ, which God gave unto him, to shew unto his servants (all of the apostles)..." The Bible doesn't record that Peter, James, or Paul "received Revelation" themselves but did they make reference to this Revelation from John in any of their pre-65 AD writings? For example: Who was Paul writing about in 2nd Corinthians 12:1-5 when he said? "...I will come to visions and revelations of the Lord: I know a man in Christ who... was caught up to the third heaven... into Paradise and heard inexpressible words,... Of such a one I will boast; yet of myself I will not boast, except in my infirmities." The context makes it clear that Paul was not referring to himself. Therefore, it seems clear from this comparison that John had this experience 14 years before Paul wrote about it.
We might consider the possibility that John had already been to Patmos; possibly several times on preaching missions, even to prisoners. The Greek intimates that John was there for the purpose of the Word and his witness of Christ and NOT as a result of the Word and His witness of Christ.