There are three types:
Fact Witness - You conducted the investigative hypnosis interview and will testify to that fact. You will testify that, yes I was called by (hiring party) to conduct an investigative interview, here are the procedures that I used, and here are the results. Opposing attorneys can call in another expert to testify as to whether you conducted the interview correctly.
Expert Witness - Someone else conducted the hypnosis interview, and you would testify on the HYPNOSIS portion ONLY, as an EXPERT.
Consulting Expert - Someone who is called in by either side to testify that the investigative hypnosis session was conducted properly. As a consultant, you must make sure that your DOCUMENTED techniques and procedures can withstand any scrutiny by the court and opposing attorneys and experts.