The elders go on to tell Paul that these Jews have been telling everyone that you teach the Jews that are among the gentiles to forsake the law of Moses (Acts 21:21). Of course they have a plan though, they tell Paul to take four men, that are under a Jewish vow and pay for their purification rites, along with doing them hisself. This way everyone will see that you still obey the law (Acts 21:22-24).
So Paul agrees to do this and when they had almost completed the week long purification some Jews noticed him and having seen him earlier with the gentile Trophimus (Acts 21:27-29) they had him arrested. It’s very important that we notice Paul was arrested for bringing a gentile into the temple, (Acts 24:6) which he did not do.
Keep in mind from this point forward, Paul has been arrested for something he did not do. So the Jews nearly rip Paul into before the Roman tribune came and arrested him(Acts 21:31-33), as he is being drug away he asks for a chance to make a defense (Acts 21:40-22:1). I love this part! Instead of standing up and pleading his innocence, Paul jumps up and starts preaching Jesus Christ brings salvation to everyone Jew and Gentile (Acts 22:2-21).
The next day Paul gets a chance to make his innocent plea to the priest and chief council. He preaches the resurrection of Jesus (Acts 23:6). Five days later he is to speak in front of the high priest, the elders, and the governor. Guess what Paul does, he preaches the resurrection of Jesus (Acts 24:1-21, Acts 24:21).
While Paul is in prison he speaks to Felix and his wife Drusilla about faith in Jesus, righteousness and the coming judgment. Think about this Paul is in prison and tells the judge that gets to sentence him that he will go to hell unless he puts faith in Jesus and repents of his sin(Acts 24:24-25).
Finally in Acts 25 Festus meets with Paul and asks if he wants a chance for defense and Paul says I’m innocent and I want this opportunity to speak to Caesar (Acts 25:11). King Agrippa shows up and Paul gets to stand in front of him. Again Paul says he is innocent but he does put forth a case to defend himself, he uses the opportunity to preach (Acts 26:1-24).
What is amazing to me is Paul is using his imprisonment to gain audiences to preach to. Paul says just enough to show that he is innocent and then moves on to preaching about Jesus.