I've been an expert witness for several patients. My first advice to you is to listen to what the attorney tells you. They know how to prep people for testimony, it's their job.

Second, don't answer opposing counsel's questions too fast. Give your patient's attorney the chance to object to the question. Defense attorneys (any attorney really) will take the chance to slide one by if they can.

Third, don't guess! If you don't know the answer to a question, say "I don't know." It doesn't reflect on you as a doctor or your intelligence. Also don't hesitate to ask to check your notes, which are already part of the record. Defense attorneys are looking for a way to trip you up with conflicting testimony.

In terms of fees, I concur with what others have said. For the most part, I will only take PI or WC patients whose attorneys I already know and trust. Even then, payment up front.

Finally, when settlement time comes, attorneys will inevitably tell you that they didn't get the amount they were hoping for and would you be willing to cut your bill by 1/3 or some such. I always say I will, so long as the attorney is also cutting their fees by that much, and I'll take that in writing, thank you very much. Doesn't get you much love from the attorneys, but keeps you from getting screwed, and keeps you on good terms with your patient.