Dr. Bill Kanasky, Jr. covers two witness deposition testimony topics: witnesses who guess and witnesses who freeze. 
1) You have to identify when a witness is guessing in their deposition. Key phrases to watch out for besides "I guess," are "I think," "I believe," "I probably," "I assume." You need to explain to your witness that even if they tell you they won't guess, their brain is wired to guess and they need to be educated on what a guess sounds like and must work with them to correct these guesses during mock deposition questioning. 
2) In addition to the well-known responses of fight or flight, there is a third response that doesn't get the same level of attention: "freeze". Referred to in psychology as "freeze appease", this survival response leads to the witness agreeing with the questioner on every question in order to appease them. When witnesses freeze, they have to appease to survive. Dealing with the freeze response requires neurocognitive training to help the witness be prepared during their testimony.
Watch the video of this episode: https://www.courtroomsciences.com/r/V51