There are two basic reasons why the opposing attorney wants to take your deposition. First, that lawyer is trying to discover information about you personally, your background, your education and work history. He is also trying to discover what information you have regarding the claim, including the facts and witnesses supporting the case.
Second, the opposing attorney is trying to pin you down or lock you into your story or testimony. If later at trial, you give an answer that is different or inconsistent with your sworn testimony, then that attorney will discredit or impeach you with your prior testimony. The theory goes that if the judge or jury does not think you are credible or mistaken on some points, then you are not reliable or honest on the key points supporting the claim.
The attorney and witness should approach the preparation for the deposition with these motives in mind. The first rule of thumb is, to tell the truth. No matter how bad the truth may seem, it is always easier to explain a truthful transgression than trying to explain why the witness was not honest. Everyone makes mistakes. Juries are more forgiving if the witness is forthcoming and honest even on difficult subjects. Also, your attorney can prepare more effectively for truthful testimony than to be caught off guard at trial with the exposure of an untruth.
Most witnesses are truthful in their testimony. However, smart lawyers can trip them up at trial when the witness attempts to remember facts on which they are not clear or not certain. Remembering times, distance and space are prime examples of areas where the witness can get in trouble. If you are not certain how fast you were going, or how many car lengths away you were, then qualify your answer by saying you are not sure or telling the lawyer not to hold you to your recollection. I do not know or I cannot remember are acceptable answers in a deposition.
The next rule in any deposition is to assume that the other lawyer either already knows the answer to the question he is posing to you, or he will get that answer before trial. Thus, if he asks you to detail all of your past medical history, be careful to qualify your answer or tell him that this is all you remember at this time. If you forget an instance where you received medical treatment in the past, the opposing lawyer will know it and bring it out at trial in an attempt to discredit you.
Another good tip to consider during your deposition is to listen carefully to the question posed by the other attorney and answer only that question. Often, witnesses will try to anticipate the next question and provide a response that they think the lawyer is asking. Long and detailed answers not only make the deposition last longer but also make it easier on the opposing attorney to discover information which he was not going to ask you.
If a question needs an answer that requires an explanation, then explain your answer. However, if the question can be responded to simply with a yes or no, then answer it in that fashion. When responding to a question that requires your best estimate, then make it clear in your answer by saying " I think it was about 3 feet." Thus, at trial, you will have some room for error if additional facts are remembered.
Finally, remember that questions are allowed in depositions that may not be allowed at trial. Thus, do not be offended or surprised about personally probing questions. They may not necessarily be asked at trial. Objections at trial are also not allowed during most depositions. Generally, only objections to the form of the question and the responsiveness of the answer are allowed during the deposition. Questions asking for hearsay or which are leading are acceptable in a deposition even though they are not allowed at trial.