Everyone on both sides rehearses testimony before a trial. While defense attorneys routinely try to hide the truth, prosecutors and detectives are trained to protect the truth. You might say that more ethical actions are being taken by the prosecution than the defense. This is because you only get one chance to say something at a trial, and saying something wrong can throw the whole trial in the wrong direction even if something is just mis-spoken.
My understanding of this practice is that they do not rehearse what is said but how it is said. This would include addressing the contations and denotations of the testimony. For example the police officer could say that the defendent was angry, outraged or infuriated. Angry has a neutral emotional conotation when compared to outraged or infuriated.
I see nothing at all wrong with this practice. The only thing that would make it wrong is if they were "practicing" not telling the truth while testifying. The practice of rehearsing testimony is not unique to prosecutors and police officers. Defense Attorney's also will do this.
Simply from the standpoint that the defense also rehearses its witnesses, it is ethical that the prosecution rehearses its--police officer or civilian. The rehearsal should, in fact, serve to make a case more ethical or credible in the sense that the prosecutor should look for inconsistencies in a witness's testimony and ask him or her to clarify those inconsistencies by sticking to the truth and evidence. After all, that is what the jury should look/listen for.
I agree with the second post. I do not see anything wrong with rehearsal as long as the point of the rehearsal is not to conceal the truth. Prosecutors should want their witnesses to emphasize the major points that they want made and they should want to help their witnesses avoid making mistakes under cross examination.
So as long as they are not coaching them to conceal any evidence, I think it is completely ethical.
I think it depends on the attitudes of the prosecutor and police officer. The trial situation is stressful, and it makes sense to practice telling the truth clearly, without getting mixed up. Without practice, most people would probably have trouble doing this. I know I would.
But if the practice sessions are used to plan to tweak the truth or obscure facts in any way, that is definitely unethical. I imagine it must be a difficult line to manage without crossing, especially considering that the officers and prosecutor are both likely to have strong beliefs about what the outcome of the trial ought to be.