Let me help lower your clients' stress level by making sure they are prepared.
Even when clients appear to be well spoken and you believe they will be fine during their deposition, they still need to be prepared. They need to understand they will be answering questions differently than they would if they were in a regular conversation. For instance, they need to be made aware that silence is fine. One tactic lawyers sometimes use is to sit quietly and stare at the person being deposed to see if they will continue talking. Witnesses should be made aware that they should not continue speaking if there is no question pending.
Clients should also be told not to joke or be facetious in a deposition because when the deposition is transcribed, it would not look like a joke.
Clients should be made aware of whether or not the deposition will be video recorded so they can dress appropriately. If the deposition is going to be video recorded, make sure your client dresses appropriately. You will also want to make sure that your client controls any nervous behaviors such as playing with hair or nervous smiling at inappropriate times. That type of preparation may take significant practice.