Expert witnesses are almost always paid for their time to appear, usually quite well.  I am assuming that you are plaintiff's treating physician and are being asked to provide expert testimony based on your treatment of the plaintiff and knowledge of his/her medical condition.  You should speak to the attorney who is requesting that you appear about being compensated for your time.  If you are subpoenaed to appear without an arrangement to be paid, you may attempt to quash the subpoena on that basis.