A first time expert witness asks: how can I extricate myself from a case without getting sued? Here is an answer from attorney Edward Hoffman:
Being an expert witness is tough — especially the first time. I happen to know a very renowned scientist who recently was deposed as an expert for the first time. My scientist friend has the kind of credentials that would floor most people, and he has a real gift for explaining complex concepts clearly. Given that he was being deposed about a question well within his expertise, I would have expected him to handle the depo in stride and I would have expected the attorney deposing him to be a nervous wreck. In fact, it was quite the opposite — simply because the experience was so new to him. My point is that your reaction to the deposition is quite normal and you should not let it affect you too much.
Backing out, if that is what you decide to do, will potentially cost you a great deal. You contracted (either orally or in writing) with this attorney to be his expert at trial, and if you do not cooperate you will be in breach of the contract. You will then be responsible for all the costs foreseeably arising out of your breach, which may be astronomical. For example, if his clients would win a million dollars with your testimony but lose without it, you will potentially be on the hook for a million dollars. Worse yet, they might sue him for malpractice as a result of your breach and he could then sue you to pay not only the damages he is required to pay out but also to reimburse him for his attorney fees defending that lawsuit.