It is hard to tell the truth. The more stressful the situation, the more difficult it is.
I do probate and elder law litigation in Oregon. One of the things lawyers do in will contests and financial elder abuse cases is take depositions. The lawyers in the case put the witnesses in front of a court reporter, make them swear to tell truth, and then ask what happened. In this way the lawyers find out what the witnesses on the other side of the case will say at trial.
To prepare my client for deposition I give some hints about how to respond to the other lawyer's questions, but most of my emphasis goes into the importance of telling the truth.
Having my client tell the truth is crucial to my case, because a client who gives one untruthful answer out of twenty casts doubt on all twenty. My clients tell me that they will tell the truth. They may even understand the importance of it to the case, but more often than not they are simply incapable of it.
In litigation the lawyers each develop a story. The challenger of the will has a story whereby the will was a result of undue influence. The proponent of the will has a story in which the will truly represents the last wishes of the person who wrote it. The judge will listen to the evidence and either accept one of the stories, or construct from what he hears a story of his own. The witnesses in deposition and trial know what story their lawyer is trying to tell and they want to help. Often, in their zeal to help, witnesses hurt their own cases.
If all the facts supported the same story, nobody would be going to court. Cases are litigated because there is a dispute as to what happened. That means some of the facts point toward the challenger's story and some of them point to the proponent. Witnesses know this and when testifying they filter their answers through the lense of how the answer fits the story their lawyer is trying to tell. When asked a question with a straightforward answer that does not fit the witnesses story, even people who are generally truthful become evasive and defensive. Evasive and defensive witnesses are bad witnesses.
Stories and real life are not the same. When we go to the movies we don't see everything every character does every minute of the day. Facts are indifferent to the stories we tell, so authors and the editors cut out anything that doesn't contribute to the story. A different author or editor would pick out different things and create a different story. In real life heros sometimes do bad things, and villains can be philanthropic. In depositions the witness may well be asked about facts that don't fit the story the witness wants to tell. Nevertheless, it is better for the case if the witness bites the bullet and tells it the way real life presented it. I tell witnesses this all the time, but for some people the story--the narrative--has become real life, and they are unable to say anything that doesn't fit.
Litigants must accept that their lawyer cannot hide the facts that don't support the case. His job is to present the facts in a way that make his client's story more likely than the one presented by the other side. When the litigants attempt to hide facts that don't support their case, they appear from the outside to be unconvinced of the story they are propounding. Secure people accept their imperfections, and secure witnesses accept that there are flaws in the story they are presenting to the court. If the witness is truthful about the weaknesses in his case, his testimony is credible on the facts that support his case. If the witness is untruthful and evase about the flaws in the case, the suspicion is that he is also untruthful and biased about the strengths of his case
It is my job as a lawyer in a will contest or elder financial abuse case to put the facts in context and perspective. My clients want to help me, but they help the most by providing me with the most accurate information possible. The same client who complains about all the work involved in obtaining and compiling medical or financial records, is often the first one to be manipulative of the few pieces of factual evidence I have to work with. In doing so this client--the one who has stood in the way of getting the information I need--devalues the little bit of factual information I have.
In a will contest the fight is always over someone else's money. The litigants didn't earn it and haven't lost it. The best witness is the one who recognizes that he or she has no moral right to the money and is willing to simply lay out the bare facts so that a judge can decided who gets it. The moment the witness decides to help by shading his testimony, he reduces the chance that the judge will decide in his favor.