A pre-trial question-and-answer session, also known as a deposition, is a critical opportunity to lock a witness into their testimony.
Not only does this question-and-answer session allow me the opportunity to find out from the witness what they did and why, but it locks them into their answers. This will be very important down the road when their case comes up for trial.
As a case approaches trial, a witness will often spend considerable time preparing for trial with their attorney. There may be instances where a witness or the person who has been sued, known as a defendant, may want to change their answer. They may not agree with the answer they gave during their pretrial testimony.
At trial, I will have an opportunity to question the same witness and ask the same questions. If the witness deviates from the answers they gave during their pretrial testimony, I can then use it to show that there are clear inconsistencies between the testimony they gave years earlier at their pretrial deposition compared to the testimony they are giving now at trial. This can have a significant effect on the witness's credibility.
A witness who is attempting to change their answer will likely give an excuse about why they are changing it. This simply provides more ammunition to show to the jury that there may be a credibility problem here.
We are permitted to use the pretrial testimony during trial to show to the witness and to the jury that when they were asked the same questions, the witness gave totally different answers. We can then explore the motive and reasoning about why the witness has changed their answers during closing argument.