You can't win a case at your clients deposition, but you CAN lose it.
I have handled depositions for other attorneys, where part of my assignment was to "prep the client". This can be a very uncomfortable situation. I have had prepped plaintiffs in a trip and fall case where they "didn't know where the accident happened". Another time the plaintiff told me "I guess I just fell". Another time, in a pedestrian knockdown case, the plaintiff told me he ran in front of a parked bus and then got nailed by a car which could not see him, because his friend had motioned to him that it was safe. What did their attorneys of record expect ME to do? What is "prep" in a case like that? I know that in those situations, the clients testimony is going to doom the case if I don't advise them properly. It's a dicey proposition..... how far you can go? I try to explain to the client the importance of the deposition, the purpose for it, and the consequences of the answers. Sometimes this is the first time the client realizes there is more to the case than "I got hurt, when do I get paid?". If I have to educate another lawyer's client about proving liability, shame on the lawyer, BUT, I will do it.
When I have depositions for my own clients, and the luxury of proper prep, here are a few helpful suggestions for proper prep:
1. I emphasize to the client that we are not there to "testify" or "prove the case" or "convince". We are there because the law gives the adversary the right to get more information about the case. However, they are going to have to work for it. Therefor, most importantly....."LISTEN TO THE QUESTION ASKED, AND ANSWER JUST THAT QUESTION". The way I illustrate this to the client is to say "If the question is.....'Did there come a time when you were involved in a motor vehicle accident', the answer to that question is 'YES'". That's it, let them ask you the details, answer the question asked and STOP.
2. THINK before you answer. The deposition is recorded on a written transcript, but the transcript will not show if you hesitate or think. If you need to think, think. If you are not sure what a question means, ask for it to be clarified.
3. Remember that the defense lawyer needs to establish what your claim IS, and what it ISN'T. Therefor, you will answer certain questions, especially about your damages or injuries, in the negative. Don't worry if you can't say yes to every question about your injuries.
4. The one time you can speak expansively is when asked about how your injuries affect you. Remember, when the questions start to be "is there anything else", your agreeing that there is nothing else will be used to limit your damages claim.
5. If there is some key element on liability, and you know the defendant will focus on it (like NOTICE in a premises cases, or damages threshold in an auto case), emphasize this to the client and go over these issues carefully. Ask sample questions and talk to the client about the answers.
6. Some clients don't know what to expect, and are so nervous they don't absorb your prep. I always explain to an inexperienced client that we will NOT be in court, there will be no judge or jury present, we will be sitting around a table, that I will be there next to you, that the deposition will not be much different than our prep, and that being nervous does not help (though I understand you may be anyway). I really work hard to calm a nervous client before we start, because these clients can fall apart in disastrous ways.
You should NOT let a case be lost by a poor deposition, and the onus is on YOU to control this.
Tomorrow, you can't lose your case taking your adversaries deposition, but you CAN win it.