指導證人之行為，被形容為是美國當事人主義制度中的黑暗秘密，有人甚至稱之為骯髒的秘密。律師在私密的情形下準備證人，不留任何紀錄，而且，所有參與者也不會公開討論這種私下的接觸。要想藉著交互詰問的功能來暴露這些不當的指導行為，其實是非常有限的。美國司法上有許多誤判的案例，造成誤判的最重要的一個原因就是檢察官的不當行為，而且，絕大部分的誤判案例，在還沒有進入司法審判程序之前，就已經發生錯誤了。也就是說檢察官在起訴前的準備證人的過程中，有不當指導證人的行為，污染了證人的陳述，以致事實真相遭到扭曲，最終導致司法誤判的不幸悲劇。辯護律師及檢察官在準備證人的過程中，如果稍有不慎，就可能讓證人受到不當影響，進而污染了證據，損害了發掘事實真相的過程。而檢察官負有憲法上以及道德上的義務以維護司法正義，因此，檢察官在準備證人以便出庭作證時，必須格外謹慎而且高度節制，必須努力追求真實事實，而不是企圖污染證人。除了檢察官主觀上會因個人目的而不當指導證人外，客觀上認知元素也會助長指導行為。認知元素包括了記憶、以及語言及溝通表達。許多檢察官對於人類記憶存取行為所具有的危險性缺乏認識，因此很可能在無意中不當影響了證人而不自知。而且，有時候連證人自己都無法察覺已經受到不當影響，以致會堅持自己虛假不實的記憶為真。另外，檢察官也可能經由某些言詞或表達的方式，向證人發出暗示或信號，最後不當影響了證人而不自知。最後，為了預防及補救檢察官從事不當指導行為所造成的傷害，有幾種措施可採行，包括了：訊問時完整記錄、交互詰問、遵行準備證人程序的行為規範、瑕疵預審、專家證言、以及訊問時全程錄影錄音。

Witness coaching has been described as the ＂dark＂ - some have even called it dirty - secret of the U.S. adversary system. It is a practice, some claim, that more than anything else has given trial lawyers a reputation as purveyors of falsehood. Witnesses are prepared by lawyers in private, no records are kept, and the participants do not openly discuss the encounter. If false or misleading testimony results, the only persons who know about it are the participants themselves. And the capacity of cross examination to expose improper coaching is extremely limited. Given its controversial nature, one would expect the practice and ethics of witness coaching to have attracted close scrutiny by courts and commentators. Interestingly, however, the subject has received relatively modest attention. A handful of judicial and ethics opinions have discussed superficially the subject of witness preparation and coaching. Practitioner manuals typically offer general guidance on how to prepare witnesses, and occasionally address tactical and ethical issues involved in coaching. Scholarly commentary has examined the ethical limits of witness preparation, particularly by differentiating acceptable techniques from improper techniques, which promote false or misleading testimony. In addition, popular culture occasionally has dramatized the subject. However, despite a discrete body of literature devoted to witness preparation generally, there has been very little discussion by courts and commentators on witness preparation and coaching by prosecutor.