Fact witnesses are not allowed to state opinions in the trial or hearing. If you happen to see some incident in the course of your practice, you may be required to testify just as if you happened to see a traffic accident while waiting for a bus. If you are a fact witness, you will be treated just like any other fact witness. That means you will probably be served with a summons and, if summoned, you must attend and answer all questions truthfully. Expert witnesses are called to give opinion evidence in their area of expertise.
Physiotherapists get to be expert witnesses in two ways.
One way is to have a patient sue someone for causing an injury that you have treated. In that case, you have a duty to assist your patient by testifying as requested even if you are not paid beforehand. You may also receive a summons for this testimony. In this case, you are free to decline any involvement in the case, and you may also require payment as a condition of your participation. If you are going to be a witness, it is advisable to insist on meeting with the lawyer who wants you to testify before the trial or hearing.