No one, at least, no attorney, is saying here that witnesses are not “prepped” for testimony. Putting a witness on the stand, or letting someone who is on your “side” of the case be called as a witness, without sitting down with them and going over what is likely to be asked, and helping them to understand how to avoid common pitfalls in testifying (like offering more than was asked), is malpractice, at best. This goes true for depositions, of course.
But “coaching” is a highly subjective term. If by it you mean, “putting words in the witness’ mouth,” I think you are going to find that almost no one does that. Partly they won’t because the canons of ethics don’t permit it, or at the least frown on it. Mostly they won’t because any attempt to put words in a witness’ mouth eventually backfires, usually spectacularly. After all, we may not be Perry Mason, but most attorneys become relatively competent at exposing untrue statements by witnesses, either directly, or by implication.
So, in the example given by Chief Pedant (who, by the way, should also pony up his credentials, that is, what part he plays in the tort system of Chicago’s courts, so we can judge his actual knowledge of witness “preparation”), while it would be perfectly legitimate to go over the facts of the situation with your client, the defendant in a homicide prosecution, you would be crossing the line into unethical behavior if you did more than to help the client see that his bare statment may not be the complete “truth.” Inducing him to make up facts, or prevaricate about the truth, would not be allowed, rarely happens in my experience (for the reasons given), and to assert that members of the profession do so in any sort of routine fashion is to cast calumny and slander upon people who, in the face of an already difficult reputation, do not deserve such treatment.
Yes, it’s an adversarial system, and yes, we have an obligation to present the “facts” in the best light possible for our clients. But the ends do NOT justify the means, which is why every damn attorney in every damn state in the union has undergone both a course on ethics and a test on ethics since the stupid bastards in Watergate proved that some people don’t remember that morality. Which, I admit, doesn’t mean that all attorneys are ethical, just the vast majority. 