He has not been called as a witness nor will he be in a courtroom with a judge. He is giving a deposition. Do not take sides, tell the truth. Review your report before you go in and you should be all right. Most likely, the attorneys and maybe even the property owner will be there to ask you questions about the report format, what you have added and adjusted for, your methodology for your adjustments and how you arrived at a final value. They may ask you about your experiece, specifically, that neighborhood, how many appraisals you have done, etc, etc, etc.
TELL THE TRUTH, THE WHOLE TRUTH AND NOTHING BUT THE TRUTH!!! But only address the questions that are asked. Otherwise, it will come back to haunt you if the opposition is trying to discredit your report. A deposition is a pre-trial exercize so that everyone will be on the same page when it DOES go to trial.