Your court rules describe how to file a notice, and issue a subpoena that compels a witnesses attendance. If it is a non-expert, you may be able to interview them, informally, before the deposition. You may get the info you need without the cost of a reporter.
You can certainly interview the potential witness, if not retained by the plaintiff as an expert, but the information you get won't do much good unless given under oath in front of a court reporter - a deposition. If the witness tells you informally that the light was green, and then later on the witness stand says that the light was red, you have no admissible evidence of the prior conflicting testimony with which to impeach the witness. Depositions can be challenging and there are a number procedural rules, but most important you need to ask a range and series of relevant and strategic questions that will pin down the deponent to prove your case or discount the other side. Suggest you consult with a litigation attorney.
Richard A. Rodgers, Esq. (805) 230-2525
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You have to call the Plaintiff (or if the Plaintiff has an attorney call him/her) and ask when the witness is available for deposition. You do this so that your request does not get continued. You need to prepare deposition subpoena and follow the California Code of Civil Procedure Sections 2020.210-2020.240. You need to have someone serve the subpoena for you and you must attach proof of service to your deposition subpoena. You must give at least 10 days notice. You can request documents produced at the deposition. If you do this, you must require 20 days notice.
If the deposition is of a party witness (such as plaintiff, PMK (person most knowledgeable at plaintiff corporation regarding a particular subject you want to take deposition of) or plaintiff's employee), you serve a notice of deposition on the opposing party. If the deposition is of a non-party, you need to serve a subpoena on the non-party witness as well. You do not need to file the notice of deposition or subpoena with the court.
The deposition notice must be served at least 10 days before the deposition if personally served and 15 days before the deposition if served by mail. (The subpoena must be served on the witness "reasonable" amount of time before the proceeding. Ten days should be sufficient.) As for the deposition itself, you need to make arrangement for a certified court reporter to be present and transcribe the proceeding.
Disclaimer: This response is for information purpose only and shall not be construed as a legal advice. This response does not create an attorney-client relationship.